High temperature solid-lubricating materials: A review

Accepted Manuscript High temperature solid-lubricating materials: A review Shengyu Zhu, Jun Cheng, Zhuhui Qiao, Jun Yang PII:

S0301-679X(18)30616-9

DOI:

https://doi.org/10.1016/j.triboint.2018.12.037

Reference:

JTRI 5537

To appear in:

Tribology International

Received Date: 13 November 2018 Revised Date:

26 December 2018

Accepted Date: 27 December 2018

Please cite this article as: Zhu S, Cheng J, Qiao Z, Yang J, High temperature solid-lubricating materials: A review, Tribology International (2019), doi: https://doi.org/10.1016/j.triboint.2018.12.037. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

High temperature solid-lubricating materials: a review Shengyu Zhu a, b, Jun Cheng a, b, Zhuhui Qiao a, b, Jun Yang a, b, * a

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China

Center of Materials Science and Optoelectronics Engineering，University of Chinese Academy of Sciences, Beijing 100049, China.

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b

* Corresponding author: Tel: +86-931-4968193; Fax: +86-931-4968019; E-mail address: [email protected]

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Abstract

This paper provides a review on the current research developments in high

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temperature solid-lubricating materials. We first briefly discuss fundamental of high temperature solid-lubricating materials, including design strategies of a low friction coefficient, a high wear resistance, a wide environment range, and how to construct high temperature solid-lubricating materials. And then the review highlights progress in the design and exploration of high temperature solid-lubricating coatings

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(monolithic coatings, multiphase coatings and self-adaptive coatings) and composites (metal, intermetallic and ceramic matrices high temperature solid-lubricating composites). Finally, we shortly introduce practical applications and trends of high

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temperature solid-lubricating materials in the future. These developments offer new avenues to both intensify the understanding of high temperature tribology and expand

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the diversity of practical applications. Keywords: Solid-lubricating materials; High temperature lubrication; Solid lubricant 1. Introduction

High temperature solid lubrication is the only viable alternative to reduce friction

in many high temperature environments, especially those that include temperatures above 350 °C because liquid lubricants degrade rapidly under these conditions [1-3]. With rapid advances in science and technology, core parts and techniques in more modern industrial tribo-systems rely on high temperature solid-lubricating materials for high performance, efficiency, and durability, especially to design and produce 1

ACCEPTED MANUSCRIPT materials that possess low friction coefficient and high wear resistance over a wide temperature range. Industrial applications in high temperature lubrication include primarily the aerospace industry (rolling element bearings, air foil bearings, gears, various satellite components) but also other industries such as the tooling, material

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forming, automotive, energy, and military industries (cylinder wall/piston ring lubrication for low-heat rejection diesel engines, small arms action components, as well as various furnace components) [4, 5].

High temperature solid lubrication remains one of the toughest challenges

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encountered in the field of tribology [6]. High temperature solid-lubricating materials, which consist of high temperature matrix materials, high temperature solid lubricants

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and some assistant components prepared by various fabrication processes, possess favorable friction-reduced and wear-resistant properties at elevated temperature. It is well known that solid lubricant is very sensitive to environment atmosphere and ambient temperature, which brings great difficulties with the design and fabrication of the advanced and universal high temperature solid-lubricating materials. Furthermore,

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in most situations, high temperature solid-lubricating materials mainly serve in extreme harsh conditions, such as alternation of atmosphere and vacuum under high temperature, high temperature corrosion environment, high speed, and heavy load

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conditions. Yet, the efforts to explore novel high temperature solid-lubricating materials possessing favorable frictional properties and superior wear resistance abilities have never stopped. As a result, great strides have been made in recent years

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in the fabrication and diverse utilization of new high temperature solid-lubricating materials that are capable of satisfying the multifunctional needs of more advanced mechanical systems.

This paper provides an overview of the current research developments in this

field, including: (1) Design strategies of a low friction coefficient, a high wear resistance, a wide environment range for high temperature solid-lubricating materials; (2) How to construct high temperature solid-lubricating materials, which is involving high temperature solid lubricants and matrix materials; (3) Progress in high temperature solid-lubricating materials, divided into high temperature coatings and 2

ACCEPTED MANUSCRIPT composites; (4) Applications of high temperature lubrication; (5) Trends of high temperature solid-lubricating materials in the future. Emphasis will be on progress in the design and exploration of high temperature solid-lubricating materials, such as NASA-PS coatings, the self-adaptive solid-lubricating coatings, the Ni matrix integrated

composites,

the

Ni3Al

intermetallic

matrix

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strength-lubricating

wide-temperature-range lubricating composites, the ZrO2 ceramic matrix high temperature lubricating composites, and the emerging lubricating materials. 2. Design of high temperature solid-lubricating materials

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2.1 Design of a low friction coefficient

A low friction coefficient is a guarantee of improving energy efficiency. At high

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temperature, many tribo-components requirement low friction coefficient to save energy or retain mobility, such as sliding bearings, forming moulds, cannon bores. On the basis of the classical theory of adhesion and solid lubrication [7-11], if chemical reaction film or physical adsorption film with low shear stress covers on the contact surface, it can achieve a low friction coefficient in boundary lubrication. During high

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temperature friction, the solid-lubricating film generated on the worn surface by physical or chemical reaction can effectively reduce the friction. The ideal design for achieving a low friction coefficient is to have an elastically stiff and hard substrate to

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support the normal load and keep the contact area small, while the surface lubricating film provides shear accommodation and reduces adhesive strength. 2.2 Design of a high wear resistance

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A long life and a high accuracy require high temperature solid-lubricating

material with a high wear resistance. High temperature wear mechanism mainly includes adhesive wear, abrasive wear, corrosive wear, and fatigue wear. The formation of solid lubricating film on the worn surface contributes to the

improvement of adhesive wear and abrasive wear. Solid lubricating film can provide a low shear film to minimize adhesive wear, and prevent the direct contact between the rubbing pair from abrasive wear. Moreover, high mechanical properties (hardness, strength, ductility, etc.) of substrate can keep off plowing of one surface by asperities on the other, or cutting action of harder particles on a softer surface, and fatigue crack 3

ACCEPTED MANUSCRIPT propagation and growth on the worn surface and subsurface. At elevated temperatures, generally, corrosive wear is dominated by oxidative wear. One hand is to prevent high temperature solid-lubricating material from oxidation; the other hand is to take full advantage of the formation of the glaze film.

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The development of the glaze film is complex, which depends on pressure of oxygen, the composition of tribo-pair, tested conditions during friction process. The completely glaze film, especially the lubricating film containing solid lubricant by tribo-oxidation reaction, is in favor of reduction to wear.

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2.3 Design of a wide environment range

High temperature solid-lubricating material mainly serves in extremely harsh

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conditions, such as atmosphere and vacuum under high temperature, high temperature corrosion environment, high speed, and heavy load conditions. Most of the thermal machinery needs to experience a low temperature startup stage or low/high temperature cycling during the operation, such as foil bearing working at a wide temperature range from cryogenic to over 650 °C. From the

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perspective of the whole service conditions, mechanical system must go through the low temperature stage during startup and shutdown periods, thus the high temperature refers to the wide temperature range from the initial environmental temperature to

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high temperature operation stage. From the engineering technology, high temperature solid-lubricating material has abilities of continuous lubrication function and satisfactory wear resistance at wide environmental range.

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Moreover, a broad vacuum range plays a critical role in mechanical systems,

such as aerocraft and satellite, which undergoes a continuous variation from air to vacuum during launch and flight or return periods. In addition, mechanical systems like underwater vehicles are confronted with alternative variation of dry and wet conditions, while stable friction coefficient over a wide contact load and speed range is crucial to safe operation of the brake material. Besides of solid lubricant being suitable for diverse environments, environmental durability should be also in consideration for design of solid-lubricating materials. Some solid-lubricating materials need to only survive a few seconds (forging dies) or 4

ACCEPTED MANUSCRIPT minutes (re-entry flight vehicle) while others must endure for years (aircraft engine parts). The lifetime of some solid-lubricating materials involve over 108 cycles (satellite parts) or several cycles (space docking mechanism). Tribological behaviors are not intrinsic properties of material relying on physical

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and mechanical properties; instead, they are systems’ properties involving interactions within pairs of contacting surfaces and between them and the environment. This dictates a need for high temperature solid-lubricating material to provide favorable friction and wear in a broad range of operating environments.

3.1 High temperature solid lubricant

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3. Architecture of high temperature solid-lubricating materials

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High temperature lubricant is a kind of lubricant that can be used to reduce friction and minimize wear between moving contact surfaces at elevated temperature, including gas, liquid or solid lubricant [12, 13]. High temperature lubricant is an entity functioning as high temperature lubricity. At high temperature, gas lubrication technology has been not mature. Due to decomposition and rapid degradation on

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exposure to the elevated temperatures, liquid lubricant is limited to less than 350 °C. However, the usable temperature of some solid lubricants can even reach 1000 °C. The use of high temperature solid lubricants is the only viable alternative to reduce

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friction in many high temperature environments, especially those that include temperatures above 350 °C.

High temperature solid lubricant has a wide variety and the complex lubrication

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mechanism, which mainly includes the following types: (1) layered structure material with weak interlayer force, such as graphite, MoS2; (2) soft metal with multiple slip planes, such as Ag, Au; (3) metal fluorides and oxides with thermal softening, like CaF2, PbO, AgMoO4.

The lubricity of solid lubricants depends closely on ambient temperature. Traditional solid lubricants with layered structure like graphite and MoS2 can provide lubrication below 400 °C due to undesirable oxidation at elevated temperatures, while fluoride graphite and WS2 can withstand temperatures up to 500 °C. Moreover, hexagonal boron nitride (hBN) can maintain lubrication at temperatures up to 1000 °C. 5

ACCEPTED MANUSCRIPT Some soft metals (e.g., Ag, Au) offer low friction at low and elevated temperatures. Metal fluorides and oxides work quite well as high temperature solid lubricants, but both fail to provide lubrication at room or lower ambient temperatures. High temperature solid lubricant can be used in form of single or composite

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lubricants for various purposes. Since there is no “universal lubricant” that can operate at a broad temperature range conditions, a synergetic/combining lubricating action, a mixture of two or more solid lubricants, is one of promising strategies to achieve high temperature solid-lubricating materials. The well-known solid lubricant

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combination is Ag and BaF2/CaF2 eutectic. Soft metal Ag works as a solid lubricant below 500 °C, whereas BaF2/CaF2 eutectic offers the lubricating function above

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450 °C. The combination of Ag and BaF2/CaF2 eutectic can provide favorable lubricity from room temperature to 1000 °C. In addition to conventional solution that lubrication is obtained by adding solid lubricant into high temperature matrix during fabrication process, another solution is in situ formation of lubricious compounds by tribo-chemical reaction during friction process. This design approach has been used in

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many materials, such as adaptive nitride-based high temperature solid-lubricating coatings and Ni3Al matrix high temperature solid-lubricating composites [14, 15]. 3.2 High temperature matrix

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Soft high temperature solid lubricant cannot carry heavy load and undergo abrasive wear. Consequently, high temperature materials need be used as matrix to bind solid lubricant and bear load. To achieve high wear resistance and low friction

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coefficient, high temperature matrix material is required to have suitable compatibility with solid lubricant, such as mechanical properties, physical properties and chemical properties. As for mechanical properties, high temperature matrix material needs to possess favorable hardness, adequate ductility and high strength to confront abrasive wear and fatigue wear. Moreover, high oxidation resistance is vital to forestall oxidative wear at high temperature. Additionally, suitable thermal expansion coefficient is also critical factor to obtain an effectively lubricating film on the worn surface. High temperature matrix material mainly includes metal, intermetallic and 6

ACCEPTED MANUSCRIPT ceramic. Furthermore, reinforcing phase is added into high temperature matrix material to enhance the hardness for metal and improve the toughness for ceramic. Sometimes, it is allowed assisting phase to adjust compatibility for tribological system, such as thermal mismatch, corrosive properties, bonding strength, and transfer effect.

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4. Progress in high temperature solid-lubricating materials Various high temperature solid-lubricating materials have been developed to meet different requirement. According to the composition, high temperature solid-lubricating material is divided into metal matrix, intermetallic matrix and

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ceramic matrix high temperature solid-lubricating material. According to the usable form, high temperature solid-lubricating material is divided into high temperature

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solid-lubricating coating/film and high temperature solid-lubricating composite/bulk. 4.1 High temperature solid-lubricating coatings

There has been considerable progress in the development of high temperature solid-lubricating coatings. Initially, high temperature solid-lubricating coatings consist of single solid lubricant or a monolithic component. However, at elevated

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temperatures, it is very difficult for single solid lubricant coatings to maintain low friction and wear simultaneously. Consequently, multiphase high temperature solid-lubricating coatings have been extensively and intensively investigated.

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Recently, on the basis of self-adaption mechanism, adaptive high temperature solid-lubricating coatings are explored, which are endowed with favorable tribological

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properties at a wide environment range. 4.1.1 Monolithic high temperature solid-lubricating coatings The monolithic high temperature solid-lubricating coatings include transitional

metal disulfides coatings, noble metal coatings, Cs-compounds coating, and sulfate coatings [16-18]. Table 1 lists tribological properties of Cs-compounds and sulfates solid-lubricating coatings. Cs-compounds coatings were reported to be very promising for lubricating Si-based ceramic components at high temperatures. Cs-compound provides Si3N4 with a good lubrication from room temperature to 750 °C, especially with an average value of 0.03 at 600 °C. During sliding at high temperature, a mixed oxide layer consisting of Cs2O and SiO2 is believed to be responsible for low friction. 7

ACCEPTED MANUSCRIPT Additionally, sulfates coatings like CaSO4, BaSO4, and SrSO4 also provide quite low friction coefficient of about 0.15 at high temperature of 600 °C [19, 20]. Tribological and mechanical properties of monolithic solid-lubricating coatings mainly depend on those of solid lubricants. Due to the sensitive nature of solid

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lubricant to environment atmosphere and ambient temperature, the monolithic high temperature solid-lubricating coatings are routinely applied under simple and specific service conditions.

Synthetic method and Materials

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Table 1 Tribological properties of Cs-compounds and sulfates solid-lubricating coatings Tested conditions and tribological results coating compositions

Si3N4 ball; 0.98 N; 0.2 m/s;

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Pulsed laser deposition

25 °C, µ: 0.27; 300 °C, µ: 0.54; 400 °C, µ: 0.30;

Cs2MoOS3 thin films

500 °C, µ: 0.16; 600 °C, µ: 0.03; 700 °C, µ:

deposited on Si3N4

0.11;

Si3N4 ball; 0.98 N; 0.2 m/s;

Cs2MoOS3 thin films

25 °C, µ: 0.50; 300 °C, µ: 0.26; 400 °C, µ: 0.31;

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Pulsed laser deposition

deposited on SiC

Pulsed laser deposition

coatings[16]

Cs2MoOS3 thin films

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Cs-compounds

500 °C, µ: 0.08; 600 °C, µ: 0.09 Al2O3 ball; 0.98 N; 0.2 m/s; 25 °C, µ: 0.30; 300 °C, µ: 0.65; 600 °C, µ: 0.12;

deposited on Al2O3

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Pulsed laser deposition Al2O3 ball; 0.98 N; 0.2 m/s;

Cs2MoOS3 thin films 25 °C, µ: 0.26; 300 °C, µ: 0.42; 600 °C, µ: 0.18

deposited on ZrO2

Pulsed laser deposition Si3N4 ball; 0.98 N; 0.2 m/s;

Cs2MoOS3 thin films 25 °C, µ: 0.67; 300 °C, µ: 0.39; 600 °C, µ: 0.18 deposited on Inconel

Pulsed laser deposition

Al2O3 ball; 0.98 N; 0.05 m/s;

CaSO4 coating

600 °C, µ: ~0.15

Pulsed laser deposition

Al2O3 ball; 0.98 N; 0.05 m/s;

Sulfates coatings[19]

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600 °C, µ: ~0.15

Pulsed laser deposition

Al2O3 ball; 0.98 N; 0.05 m/s;

SrSO4 coatings

600 °C, µ: ~0.15

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4.1.2 Multiphase high temperature solid-lubricating coatings The multiphase high temperature solid-lubricating coatings were pioneered by USA National Aeronautics and Space Administration (NASA) laboratories. In the past 40 years, the PS100, PS200, PS300 and PS400 families of plasma sprayed

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coatings with solid-lubricating behavior were developed at NASA Lewis Research Center (now named NASA Glenn Research Center) [21-28]. Comparison of NASA

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plasma spray coatings is shown in Table 2.

The chief design concept of PS high temperature solid-lubricating coatings is to employ the combination of solid lubricants in obtaining acceptable lubricity over a wide temperature range because the single solid lubricant cannot achieve low friction coefficient from low temperature to elevated temperature. It is found that the joint

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action of silver and fluorides is a most effective combination. The PS100 family consisting of nickel-glass-solid lubricant coatings is the first generation PS high-temperature solid-lubricating coating. As PS100 coating worked on the bearing

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for a long time, it exposed the following shortcoming: low hardness, high wear rate, and easy oxidation of above 700 °C. Subsequently, to remedy the weakness, PS200

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coating system were developed, which are composed of a hard nickel-cobalt-bonded chrome carbide matrix and solid lubricants of Ag and BaF2/CaF2 eutectic. However, PS200 have some drawbacks. The hard nickel-cobalt bonded chrome carbide requires diamond grinding prior to service. Furthermore, at high temperatures above 800 oC in air, chrome carbide oxidizes to cause slight dimensional swelling of the coatings. To overcome these disadvantages, PS300 coatings supplanted the nickel-cobalt bonded chrome carbide of PS200 coatings with the nickel-chrome bonded chrome oxide. This coating system was not very hard but had desirable wear resistance and friction coefficient, especially at elevated temperature up to 650 °C. Recently, a new coating 9

ACCEPTED MANUSCRIPT system, PS400 containing a nickel-molybdenum-aluminum matrix with chrome oxide hardeners combined with silver and fluoride solid lubricants, has been developed due to several defects of PS300, namely the need to undergo a heat treatment for dimensional stabilization and poor initial surface finish.

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These four distinct families of coatings have engineering application over the last four decades. PS200 has solved a wide range of lubrication problems in prototype hardware applications such as process control valve stems, foil air bearings, rotating face valves and butterfly valve stems. Additionally, PS200 has been used successfully

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in a Stirling engine as a cylinder wall lubricant coating and also as a gas bearing journal back up lubricant. PS304, one of the best performing PS300 coatings, is

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comprised of 20 wt.% Cr2O3, 10 wt.% Ag, 10 wt.% BaF2/CaF2 eutectic, and 60 wt.% Ni-Cr matrix, which is successfully applied to foil gas bearings and other high temperature sliding contacts.

Table 2 Comparison of NASA plasma spray coatings [21-30] Tested conditions

Binder

designation

matrix

Harder

NiCr

General attributes

Glass

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PS200

PS300

NiCo

NiCr

and tribological

lubricants

Ag-Fluorides

results

Soft-high wear

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PS100

Applications

Solid

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Coating

Cr3C2

Cr2O3

Ag-Fluorides

Ag-Fluorides

NiCr pin; 5 N; 1.0

compressor/turbine

m/s; µ: 0.15-0.25;

shaft seal

W: 10-3 mm3/Nm 5 N, RT-850°C; µ:

a cylinder wall in a

(abrasive to counter

0.29-0.38;

stirling engine

face dimensionally

W:

stable)

mm3/Nm

Moderate hardness,

500 °C, µ: 0.25;

foil gas bearings;

mildly abrasive to

W: 280±30 × 10-6

steam

counter face, poor

mm3/Nm

governor valve lift

dimensional

650 °C, µ: 0.23;

rods

Hard-low

wear,

10-5-10-6

turbine

10

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NiMoAl

Cr2O3

Ag-Fluorides

W: 100±10 × 10-6

heat treatment

mm3/Nm

Excellent

500 °C, µ: 0.16;

dimensional

W: 6.3±1.0 × 10-6

stability and surface

mm3/Nm

finish, poor initial

650 °C, µ: 0.21;

low

W: 7.6±1.2 × 10-6

temperature

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PS400

stability-requires

mm3/Nm

tribology

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RT: room temperature; µ: friction coefficient; W: wear rate; PS: plasma sprayed

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NASA PS coatings firstly realize the excellent lubricity over a wide temperature range. Base on NASA PS coating idea and strategy, some new solid-lubricating coatings developed by adjusting composition or altering preparation technique attempt to improve the overall performance, and some evolutions of solid-lubricating coating and preparation technology are presented as shown in Table 3 [31-39].

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The plasma spraying composite coatings usually consist of porous microstructure and low cohesive strength, which causes the reduction in the hardness and bonding strength of the coatings. These disadvantages can be overcome by an advanced

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surface modification technology-laser cladding, which uses a high power laser beam to form a coating metallurgically bonded to the substrate. Referring to PS212 coatings,

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a NiCr-Cr3C2-Ag-BaF2/CaF2 high temperature solid-lubricating coating was prepared by laser cladding [31, 32]. The coating exhibits high microhardness and metallurgically bonding strength, and low friction coefficient of 0.28-0.36 from room temperature to 500 °C when rubbing against Si3N4 ball. On the base of PS304 coatings, a PM304 coating on a Ni-based superalloy rod

prepared by high-energy ball milling and powder metallurgy techniques shows the dense microstructure of PM304 coating and refinement of solid lubricants [36-39]. The PM304 coating displays a relative density of 93 % and a high tensile strength of about 46 MPa. The tensile fracture occurs not only in the interface between coating 11

ACCEPTED MANUSCRIPT and substrate, but also appears in the coating inside. The high tensile binding strength of the PM304 coating is mainly due to the increase of density, the decrease of sharp pores number, a metallurgical bonding formed between Ag and NiCr, and the dispersion hardening effect of fine particles precipitated from the NiCr substrate.

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According to PS400 coatings, the NiMoAl-Ag coating by high velocity oxygen fuel (HVOF) spraying exhibits a highly dense microstructure and bonds well with the substrate [35]. The results show that the friction coefficient is approximately 0.30 from 20 °C to 600 °C and reaches the lowest value of 0.09 at 800 °C. Meanwhile, the

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wear rate is maintained on the order of 10-5 mm3/Nm at the test temperatures except for 400 °C and 600 °C. The lubrication mechanism of the NiMoAl-Ag composite

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coating depends on testing temperature. Below 400 °C, silver is enriched on the sliding surface to form a continuous lubrication film. At 600 °C and 800 °C, silver molybdates produced by tribo-chemical reactions can form a glaze layer to reduce the friction and wear at high temperatures. In addition, at 800 °C, silver that can diffuse and coalesce on the wear scar in a molten state further reduces the friction and wear of

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the coating with a synergistic lubricating effect of Ag2MoO4. To further improve wear resistance of the coating, the HVOF-sprayed NiMoAl-Ag-Cr3C2 composite coating exhibits low friction coefficient below 0.30 between room temperature and 800 °C

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and wear rate at the order of 10-6 mm3/N·m except 400 °C [40]. Since friction components are often exposed to cycling conditions with alternate stages of room temperature and high temperature operation, it is reported that the atmospheric plasma

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sprayed NiMoAl-Al2O3-Ag composite coatings also shows solid-lubricating properties in reversible temperature cycles [41]. To broaden the usable temperature of PS coating, NiCrAlY alloy is selected as

the binder of high temperature coating due to high temperature oxidation resistance, instead of NiCr alloy. The NiCrAlY-Cr2O3-Ag-CaF2/BaF2 high temperature coating prepared by plasma spray shows that the coating exhibits friction coefficient of below 0.40 and wear rate of about 10-5 mm3/Nm from room temperature to 1000 °C. The NiCrAlY-Ag-Mo composite coating prepared by atmospheric plasma spraying exhibits low friction coefficient around 0.30 and wear rate at the order of 10-5 12

ACCEPTED MANUSCRIPT mm3/Nm from 20 °C to 800 °C, while the nanostructured composite coating presents higher microhardness

and adhesive strength [42-44]. The laser cladding

NiCrAlY-Cr3C2(NiCr)-V2O5-Ag2O solid-lubricating coating shows that the friction coefficient is as low as 0.14 and the wear rate maintains 2.9 × 10-5 mm3/Nm at 800 °C,

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which can be attributed to formation of Ag3VO4 and AgVO3 with lamella structure on the worn surface. The laser cladding NiCrAlY-Ag-MoO3 coating offers the friction coefficients of 0.24 and 0.23 at 600 °C and 800 °C, respectively, while the NiCrAlY-Cr3C2(NiCr)-Cu-MoO3 solid-lubricating coating has the friction coefficient

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0.34 and the wear rate was 1.0 × 10-5 mm3/Nm at 800 °C. Additionally, it is also reported for a NiCoCrAlY-WSe2-BaF2/CaF2 coating prepared by plasma spraying and

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a self-lubricating NiCoCrAlY-Cr2O3-Ag-Mo composite coating deposited by atmospheric plasma spray technology [45, 46].

Besides PS coatings, NASA also investigated ceramic matrix lubricating coatings. The results shows that friction coefficients are 0.40 at room temperature and 0.22 at 650 °C for the ZrO2-CaF2 coating, and Ag addition has the detrimental effect of

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improving room temperature friction and wear for the ZrO2-CaF2-Ag coating [2]. Currently, many researches indicate that ZrO2 matrix lubricating coatings have attractive tribological properties from room temperature to 800 °C [47-51].

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The combination of Ag and fluorides is widely investigated at various high temperature lubricating coatings, but other solid lubricants at a broad temperature range are rarely reported. It should be noted that hBN with layered structure like

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graphite has many advantages, such as high thermal stability, good chemical inertness, and high thermal conductivity. However, the non-wettability and poor sintering ability of hBN restrict its applications. The electroless plated Ni-coated hBN particulates improve the wettability with stainless steel and sinter ability as well. With the assistance of sintering method, Ni-hBN coating on the stainless steel substrate was prepared by laser cladding technique [52]. The coating offers high microhardness and favorable friction-reducing and anti-wear abilities at elevated temperatures up to 800 °C, which could be attributed to good solid lubricating performance of hBN. Additionally, it is also reported for a Ni3Al-hBN-Ag composite coating on the 13

ACCEPTED MANUSCRIPT Ni-based superalloy substrate prepared by reactive sintering method and a NiCr/Cr3C2-NiCr/hBN composite coating prepared by plasma sprayed method [53, 54]. It is no doubt that multiphase high temperature solid-lubricating coatings make

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considerable progress. Lubrication over a wide temperature range from room temperature to 1000 °C is achieved. Additionally, some multiphase solid-lubricating coatings meet the requirement of high temperature lubrication technology, such as PS200 coating for Stirling engine lubrication, PS304 coating applied to foil gas

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bearings. Nevertheless, the further researches still are intensified, such as lubrication and wear mechanisms at high temperature, high temperature oxidation resistance, and

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mechanical strength.

Table 3 Tribological properties of other multiphase coatings Synthetic method and coating Materials

Tested conditions and tribological results

compositions

Si3N4 ball; 5 N; 0.113 m/s; RT-500 °C, µ:

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Laser cladding

NiCr-Cr3C2-Ag-BaF2/CaF2[31]

0.28-36; W: 10-5 mm3/Nm Inconel X-750; 9.8 N; 1 m/s;

PM

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Other

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NiCr-Cr2O3-Ag-BaF2/CaF2 [38]

multiphase

W:10-4 mm3/Nm Si3N4 ball; 5 N; 0.1 m/s; RT-600 °C,

HVOF

NiMoAl-Ag

20-800 °C, µ: 0.32-0.41;

µ:~0.3; 800 °C, µ: ~0.09; W: 10-5

[35]

mm3/Nm

coatings

HVOF

Si3N4 ball; 15 N; RT-800 °C;

NiMoAl-Ag-Cr3C2 [40]

µ: ~0.3; W: 10-6 mm3/Nm (except 400 °C)

atmospheric plasma Al2O3 ball; 12 N; 0.1 m/s; RT-900 °C; spraying NiMoAl-Al2O3-Ag

[41]

atmospheric plasma

µ: 0.53-0.17; W: 1.47-8.84× 10-5 mm3/Nm Al2O3 ball; 10 N; 0.3 m/s; RT-900 °C; 14

ACCEPTED MANUSCRIPT µ: 0.62-0.18; W: 6.0-9.0× 10-5 mm3/Nm

spraying NiAl-Mo-Ag [55] atmospheric plasma

Al2O3 ball; 10 N; 0.3 m/s; RT-900 °C; NiAl-Cr2O3-Mo-Ag

[55]

Si3N4 ball; 5 N; 0.3 m/s; RT-800 °C; µ:

NiCrAlY-Ag-Mo [44]

~0.3; W: ~10-5 mm3/Nm

Laser cladding

Si3N4 ball; 3 N; 0.19 m/s; 800 °C; µ:

NiCrAlY-Cr3C2(NiCr)-V2O5-Ag2O[56]

~0.14; W: 2.86 × 10-5 mm3/Nm

SC

HVOF

NiCrAlY matrix

µ: 0.62-0.18; W: 2.0-3.0× 10-5 mm3/Nm

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spraying

Laser cladding

Si3N4 ball; 3 N; 0.19 m/s; 600 °C; µ:

NiCrAlY-Ag-MoO3[57]

~0.24; 800 °C; µ: ~0.23;

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coatings

Laser cladding

Si3N4 ball; 3 N; 0.19 m/s; 800 °C; µ:

NiCrAlY-Cr3C2(NiCr)-Cu/MoO3[58]

~0.34; W: 1 × 10-5 mm3/Nm

Atmospheric plasma spray

Si3N4 ball; 10 N; 0.5 m/s; 500 °C; µ: ~0.3;

NiCoCrAlY-Cr2O3-Ag-Mo [46]

W: ~10-5 mm3/Nm

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Plasma spraying ZrO2-CaF2[2]

RT, µ: 0.4; 650 °C, µ: 0.22 Al2O3 ball; 20-800 °C;

Low pressure plasma spraying

coatings

ZrO2(Y2O3)-CaF2[48]

EP

ZrO2 matrix

RT, µ: 0.63-1.15; 50 N, 600-800 °C, µ: 0.48-0.57 Al2O3 ball; 20-800 °C;

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Low pressure plasma spraying ZrO2(Y2O3)-Cr2O3-CaF2[49]

50 N, µ: 0.4~0.8; 80 N, µ: 0.35~0.75;

Low pressure plasma spraying

Al2O3 ball; 30-80 N; RT-800 °C; µ:

ZrO2(Y2O3)-CaF2-Ag2O[47]

0.44-0.8

Reactive sintering

Si3N4 ball; 5 N; 0.13 m/s; RT-800 °C; µ:

hBN

Ni3Al-hBN-Ag [53]

0.32-0.48; W: 2.3-5.2 × 10-5 mm3/Nm

coatings

Plasma spraying

Si3N4 ball; 10 N; 0.188 m/s; RT-800 °C;

NiCr/Cr3C2-NiCr/hBN [54]

µ: 0.55-0.6; W: ~10-5 mm3/Nm 15

ACCEPTED MANUSCRIPT Laser cladding

Si3N4 ball; 100 N; 0.226 m/s; RT-800 °C;

Ni-hBN [52]

µ: 0.46-0.26; W: 1 × 10-7 g/Nm

4.1.3 Self-adaptive high temperature solid-lubricating coatings

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USA Air Force Research Laboratory (AFRL) has synthesized and evaluated a number of adaptive high temperature solid-lubricating coatings, also named “chameleon” coatings, which are as a new class of smart materials that are designed to adjust their surface chemical composition and structure in the various working

SC

environment to reduce friction and wear between contact surfaces. Adaptive oxide-based and nitride-based high temperature solid-lubricating coatings were

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developed progressively. The hard matrices were incorporated into soft metal for low-moderate temperatures and dichalcogenide and carbon phases for low temperature lubrication, while lubricous oxides provide low friction coefficient at high temperature [59-63]. Tribological properties of the typically adaptive coatings are given in Table 4.

YSZ-Au,

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Adaptive oxide matrix high temperature solid-lubricating coatings, such as YSZ-Ag-Mo,

YSZ-Ag-Mo-MoS2,

YSZ-Au-DLC-MoS2

and

Al2O3-DLC-Au-MoS2 coatings, were produced by hybrid magnetron sputtering and

EP

pulsed laser deposition processes [60, 63-66]. The friction coefficients are in the range from 0.10 to 0.40 for 5000 to 10000 cycles from room temperature to 500 °C.

AC C

However, soft metal diffusion on the surface reduces the lifetime of coatings. By tailoring the pattern in the TiN diffusion barrier, the diffusion of silver to the wear scar is restricted, therefore providing a larger supply of soft metal. The coating longevity is extended without sacrificing low friction properties. Adaptive nitride-based high temperature solid-lubricating coatings was explored

continuously, which include Mo2N-Ag [67], Mo2N-MoS2-Ag [59, 61], Mo2N-Cu [68], TiN-Ag [69], CrN-Ag [70], ZrN-Ag [71], CrAlN-Ag [72], VN-Ag [73], NbN-Ag [74], and TaN-Ag [75]. At low-moderate temperatures, soft metal with an easy shear can effectively lubricate for nitride-based hard coatings using metal diffusion on the worn 16

ACCEPTED MANUSCRIPT surface. When the working temperature exceeds 500 °C in air, tribo-oxidation is a dominating lubricating mechanism. The frictional surfaces of the composite coatings form the lubricious ternary oxides such as silver molybdates, vanadates, niobates, and tantalates. It is noted that the lubricious ternary oxides are found to be associated with

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the layered atomic structure with the weak interplanar bonds. When tested against silicon nitride at high temperatures, the friction coefficient for silver molybdates in Mo2N-MoS2-Ag coating is found to be in the 0.10-0.20 range in the vicinity of 600 °C, silver vanadates in VN-Ag coating has the friction coefficient of 0.15-0.20 in the

SC

700-1000 °C range, while silver niobates in NbN/Ag and silver tantalates in TaN/Ag coating has the average friction coefficient of 0.27 and 0.23 at 750 °C, respectively.

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Based on the find of lubricious ternary oxides, the individual lubricant coatings like the Ag3VO4 coating and the AgTaO3 coating were also investigated [76-79]. It was found that the silver tantalate coating against Si3N4 counterface reveals very low friction coefficients in the 0.04-0.15 range at 750 °C.

Self-adaptive coatings provide an inspiring clue to explore smarting high

TE D

temperature lubricating materials. At a wide temperature range, adaptive solid-lubricating coatings offer low friction coefficient in the 0.10-0.40 range, especially at elevated temperatures, providing friction coefficient below 0.20. The

EP

broad temperature range adaptation are attributed to adaptive mechanisms using metal diffusion, adaptive mechanisms using tribo-oxidation, and adaptive mechanisms using structural transitions. The synergistic action of these mechanisms can cover the full

AC C

range. Special emphasis will be placed on modern practices that are aimed at enhancing the properties of these coatings and expanding their uses in practical applications.

Table 4 Tribological properties of the self-adaptive coatings Synthetic method and coating

Tested conditions and

composition

tribological results

Oxide-based lubricating

MP/PLD

Sapphire ball; 0.2 m/s;

coatings

ZrO2(Y2O3)-Au[62]

25 °C, µ: 0.15; 500 °C, µ: 0.2;

Materials

17

ACCEPTED MANUSCRIPT HFV/MP/PLD

Si3N4 ball; 1 N; 0.2 m/s;

ZrO2(Y2O3)-Ag-Mo[63, 65]

25-700 °C; µ 0.3-0.4 Si3N4 ball; 1 N; 0.2 m/s;

HFV/MP/PLD ZrO2(Y2O3)-Ag-Mo-MoS2[64]

RT, µ: 0.2; 300°C, µ: 0.1;

HFV/MP/PLD ZrO2(Y2O3)-DLC-Au-MoS2[80]

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500°C, µ: 0.2; 700°C, µ: 0.2 Si3N4 ball; 100 g;

500 °C, µ: 0.2-0.25

Si3N4 ball; 100 g; 0.2 m/s;

Al2O3-DLC-Au-MoS2[60]

500 °C, µ: 0.1-0.2

SC

HFV/MP/PLD

Si3N4 ball; 1 N; 0.2 m/s; 10000 cycle,

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UMS Mo2N-MoS2-Ag[61]

350 °C, µ: 0.37-0.45, 600 °C, µ: 0.10-0.45

Si3N4 ball; 2 N; 0.11 m/s;

UMS

VN-Ag[73]

750°C, µ: 0.10; 1000°C, µ: 0.20

AC C

EP

TE D

Nitride-based lubricating coatings

RT, µ: 0.35; 350°C, µ: 0.30;

UMS

Si3N4 ball; 1 N;

NbN-Ag[74]

RT-1000 °C, µ: 0.27-0.35 Si3N4 ball; 1 N;

UMS

NbN-Ag-MoS2[74]

RT, µ: 0.27; 350°C, µ: 0.29; 750°C, µ: 0.06; 350°C, µ: 0.4

UMS TaN-Ag[75]

RT-750 °C, µ: 0.39-0.23

HFV/MP/PLD: hybrid filtered vacuum arc/magnetron sputtering/pulsed laser deposition;

UMS/MS: (unbalanced) magnetron sputtering

4.2 High temperature solid-lubricating composites High temperature solid-lubricating composite plays a critical role in progress in industrial development. In early stage, the investigation of solid-lubricating composite 18

ACCEPTED MANUSCRIPT focused on bronze alloy, Babbitt alloy and graphite material. With the development of industrial technology, these traditional solid-lubricating materials are unacceptable due to low permissible stress and allowable temperature. Therefore, it is addressed on high temperature lubrication of Fe matrix and Ni matrix composites. Moreover, in

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extreme operation environment of aerospace and nuclear field, the mechanical, environmental, and endurance requirements exceed the available high temperature lubrication and wear resistance technologies. This demands novel high temperature solid-lubricating

composite,

such

as

high-performance

structural-lubricating

SC

integrated composite, wide environment range lubricating composite, super-high temperature lubricating composite. Aiming to the above requirements, many efforts

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are made to investigate a series of high temperature solid-lubricating composites including metal matrix, intermetallic matrix and ceramic matrix solid-lubricating composites.

4.2.1 Metal matrix high temperature solid-lubricating composites Metal matrix high temperature solid-lubricating composites are mainly

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consisting of Al matrix, Cu matrix, Fe matrix, Ni matrix, Co matrix solid-lubricating composites. In general, Al matrix and Cu matrix solid-lubricating composites are viable candidates for the employment below 400-500 °C [81-85], while the

EP

permissible temperature of Fe matrix solid-lubricating composites is at most 700 °C [86-88]. Ni matrix and Co matrix solid-lubricating composites have high usable temperature, but not exceeding 900 °C for long term applications [89-91]. Table 5

AC C

lists tribological properties of metal matrix high temperature solid-lubricating composites.

Al alloys have been widely used for applications in automobile and aeronautic

components owing to their light weight, high specific strength and good wear resistance. The tribological properties of Al alloys at room temperature have been extensively studied, while the research about their high temperature wear behavior is limited. Lately, the result indicates that Al-20Si-5Fe-2Ni alloy has a favorable wear-resistant performance from room temperature to 300 °C [92]. Further researches show that the Al matrix composites with addition of Cu-coated graphite can offer low 19

ACCEPTED MANUSCRIPT friction and prevent severe adhesive wear at 500 °C, resulting from the combined effect of solid lubrication provided with graphite and dispersion strengthening by in-situ formed Al2Cu intermetallic [83, 84]. Cu alloys and composites are the candidate for solving oil-free lubrication

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problem below 400-500 °C, which have been widely used as sliding bearings and sleeves in many industries due to their good tribological properties as well as remarkable corrosion resistance and electric conduction. Soft metal with low melting point and graphite with layer structure usually work as solid lubricants for Cu matrix such

as

Cu-Sn-Zn-Pb-graphite

composite,

Cu-Ni-Fe-Sn-graphite

SC

composites,

composite [81, 82].

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The research of Fe matrix high temperature solid-lubricating composites focuses on the influence of solid lubricant including graphite, CaF2, PbO and MoS2 [86-88]. Graphite has been widely used in Fe matrix solid-lubricating composite due to the excellent lubricating performance at low temperature. Moreover, carbon can be dissolved in austenite to enhance the stability of austenite since the diffusion of

TE D

carbon atoms shortens the pearlite incubation period and promote the transformation of austenite. Usually, refractory metal molybdenum works as a reinforced phase of Fe-graphite solid-lubricating material. During the sintering, graphite reacts with Mo to

EP

form the stable and hard carbides, causing to the increase in deformation resistance of the matrix and the improvement of wear resistance of the composite. Some other alloying elements are also added to Fe matrix to enhance high temperature

AC C

performance of solid-lubricating material, such as Ni, Cu, etc. Currently, a great deal of research work is addressed on Ni matrix high

temperature solid-lubricating composites. The effects of solid lubricant (e.g., graphite, MoS2, hBN, Ag, PbO, SrSO4, Ag2MoO4, Ag2Mo2O7, BaMoO4, BaCr2O4, CeF3, CaF2/BaF2) and Ni matrix composition (e.g., NiCr, NiCo, NiMoAl) on tribological behavior are intensively studied [93-116]. In the past years, a series of solid-lubricating composites based on nickel alloys have been developed by powder metallurgy methods, such as PM212 [29, 30], PM300 [117], Nickel alloy-graphite-Ag [90], Nickel alloy-WC-Co-Mo-PbO [93], Nickel alloy-Ag-CeF3 [105], Nickel 20

ACCEPTED MANUSCRIPT alloy-graphite-CeF3 [102], NiCr-Al2O3-SrSO4-Ag [114], Nickel alloy-MoS2-graphite [97], etc.. Of these materials, PM212 shows a promise for use over a wide temperatures range from room temperature to 900 °C, another PM300 possesses well tribological behavior from room temperature to 650 °C, however, above 800 °C, the

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decline in mechanical properties degrades its wear resistance. Lately, some lubricating oxides are attracted more and more attentions due to desired lubricating properties at elevated temperature. Regrettably, unlike to metal fluoride lubricant with chemical stability, the lubricous oxides added into matrix phase are inclined to decompose

SC

during high temperature fabrication process, consequently leading to the inexistence of solid lubricant. It seems to be an infeasible strategy to achieve desirable lubrication

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by direct addition of lubricous oxides into high temperature matrix. Although many efforts are made to explore new solid-lubricating composites, tribological properties at a broad temperature range are inferior to those of PM212 and PM300. For high temperature solid-lubricating composite, there is a common issue that is a balance between tribological properties and mechanical properties; that is, solid

TE D

lubricant added bulk composites can reduce composite’s strength markedly. Therefore, it is a major challenge to produce the simultaneously superior tribological properties and high strength of solid-lubricating composites in bulk form useful over a wide

EP

temperature range for making load-bearing parts required lubrication. Recently, Ni matrix composites integrated lubrication and strength have been developed by powder metallurgy methods [118-125]. Particularly, the composite represents remarkable

AC C

solid-lubricity (µ < 0.25) over a wide temperature range (25-800 °C) and coexisting high strength (at 800 °C: compressive stress of 500 MPa), as shown in Fig. 1 [125]. It should be noted that both at air and in vacuum, the Ni matrix solid-lubricating composite exhibits satisfied friction and wear properties at elevated temperatures. Moreover, it is found that friction coefficient is superior in vacuum than at air from room temperature to 800 °C, which is the contribution of the lubricating Ag film in vacuum [118, 119]. Schematic diagrams on the friction and wear mechanism of the composite under vacuum and air conditions are revealed in Fig. 2 [118]. To explore and obtain better solid lubricity, further investigations reveal that the appropriate 21

ACCEPTED MANUSCRIPT alloying elements not only promote the mechanical properties but also improve the tribological behavior; the synergistic action of solid lubricants plays a crucial role in the improvement of lubrication over a wide temperature range. The material with the simultaneously excellent lubricity and high strength might open new applications in

1400

0.9

In Air

1200

Counterface: Inconel 718 Speed: 1 m/s Load: 5 N

0.7

1000 Stress (MPa)

Friction coefficient

0.8

In Vacuum

0.6 0.5 0.4 0.3 0.2

600 400 200

0.1 0

200

400

600

Temperature (°C)

0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

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0.0

800

o

800 C o 600 C o 400 C o 200 C

SC

1.0

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high temperature tribology.

800

Strain

Fig. 1 Friction coefficients of Ni matrix high temperature solid-lubricating composite in air and vacuum from room temperature to 800 °C; compressive properties of Ni matrix high temperature

EP

TE D

solid-lubricating composite from 200°C to 800 °C [125]

AC C

Fig. 2 Schematic diagrams on the friction and wear mechanism of the composite under vacuum and air conditions [118]

Table 5 Tribological properties of metal matrix high temperature solid-lubricating composites Tested conditions and tribological

Materials

Synthetic method and materials composition results Al2O3 ball; 3 N; 0.2 m/s;

Al matrix composites

Spark plasma sintering [84]

Al-Si-Fe-Ni-Graphite

RT-500 oC, µ: 0.3-0.4, W: ~10-5 mm3/Nm 22

ACCEPTED MANUSCRIPT Al2O3 ball; 3 N; 0.2 m/s; Spark plasma sintering

RT-350 oC; µ: 0.34-0.46; W:

[85]

Al-Fe-V-Si-Graphite

4.36-9.75 × 10-4 mm3/Nm 45 # steel; 39.2 N; 1 m/s;

Hot pressing sintering [81]

Cu-Sn-Zn-Pb-Graphite

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RT, µ: ~0.14, W: 4 × 10-6 mm3/Nm

Cu matrix

450 oC, µ: ~0.15, W: 7 × 10-4

composites

mm3/Nm

[82]

Cu-Ni-Sn-Fe-Graphite

SC

Cr12 steel; 26 N; 0.51 m/s; Hot pressing sintering

RT~500 oC; µ: ~0.2; W: ~10-10

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mm3

5140 steel pin; 39.2 N; 1 m/s;

Hot pressing sintering

RT, µ: 0.36-0.38, W: 8 × 10-5

Fe-Mo-CaF2[86]

mm3/Nm; 600 oC, µ: 0.28-0.30, W: 1.8 × 10-6 mm3/Nm

Fe matrix

TE D

5140 steel pin; 39.2 N; 1 m/s; RT, µ: 0.36, W: 3.13 × 10-6

Cold pressing and sintering composites

[87]

Fe-Mo-Ni-Graphite

mm3/Nm; 320 oC, µ: 0.36, W:

EP

3.29 × 10-6 mm3/Nm; 450 oC, µ: 0.32, W: 6.44 × 10-7 mm3/Nm 440 C steel ball; 20 N; 0.3 m/s;

AC C

Cold pressing and sintering

Fe-Ni-Cu-Cr-W-WS2-PbO

[88]

RT-600 oC, µ: 0.3-0.5, W: ~10-6 mm3/Nm; R41 alloy disk; 4.9 N; 2.7 m/s; RT, µ: 0.35±0.05, W: 3.2±1.5 ×

Cold isostatic pressing and sintering Ni matrix PM212, Ni/Co/Cr3C2-15wt% Ag - 15wt% composites BaF2/CaF2

[29]

10-5 mm3/Nm (PM212 pin), 7.2±2.0 × 10-5 mm3/Nm (R41 disk); 350 oC, µ: 0.38±0.02, W: 3.9±1.8 × 10-5 mm3/Nm (PM212 23

ACCEPTED MANUSCRIPT pin), 3.5±1.0 × 10-6 mm3/Nm (R41 disk); 760 oC, µ: 0.35±0.06, W: 3.6±0.9 × 10-6 mm3/Nm (PM212 pin), 1.0±0.6 × 10-5

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mm3/Nm (R41 disk); 850 oC, µ: 0.29±0.03, W: 4.1±2.0 × 10-6

mm3/Nm(PM212 pin), 5.0±1.0 × 10-6 mm3/Nm (R41 disk)

SC

R41 alloy disk; 4.9 N; 2.7 m/s; RT, µ: 0.37±0.04, W: 1.8±0.5 ×

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10-5 mm3/Nm (PM212 pin),

0.45±0.11 × 10-5 mm3/Nm (R41 disk); 350 oC, µ: 0.32±0.07, W:

2.5±0.3 × 10-5 mm3/Nm (PM212

Hot isostatic pressing

PM212, Ni/Co/Cr3C2-15wt% Ag - 15wt%

TE D

BaF2/CaF2

[30]

(R41 disk); 760 oC, µ: 0.31±0.04, W: 0.7±0.4 × 10-6 mm3/Nm (PM212 pin), 2.2±0.8 × 10-5 mm3/Nm (R41 disk); 850 oC, µ: 0.29±0.04, W: 8.3±1.0 × 10-6

EP AC C

pin), 0.85±0.4 × 10-6 mm3/Nm

mm3/Nm(PM212 pin), 0 mm3/Nm (R41 disk)

Hot pressing sintering

low carbon steel 1011; 100-400

PM300, NiCr-20wt%Cr2O3-10wt% Ag -

N; 0.183-0.732 m/s; 540 oC, µ:

10wt% BaF2/CaF2[117]

~0.3 TZM alloy disk; 20 MPa; 10 m/s;

Medium frequency induction heating Ni alloy-graphite-Ag

[90]

Hot pressing sintering

RT-700 oC, µ: 0.18-0.31, W: ~2 × 10-6 mm3/Nm Hastelloy C pin; 39.2 N; 0.5 m/s; 24

ACCEPTED MANUSCRIPT Ni alloy-Ag-CeF3[105]

RT-700 oC, µ: 0.11-0.25 Hastelloy C pin; 49 and 98 N; 1.5

Hot pressing sintering

m/s;

Ni alloy-graphite-CeF3[102]

RT-700 oC, µ:~0.3, W: ~10-5

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mm3/Nm Si3N4 disk; 50 N; 0.8 m/s; Hot pressing sintering Ni-Cr-W-Fe-C-MoS2

RT-600 oC, µ: 0.14-0.27, W:

[97]

1.0-3.5 × 10-6 mm3/Nm

SC

Al2O3 ball; 10 N; 0.1 m/s;

Hot pressing sintering

3.19-0.546 × 10-5 mm3/Nm

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NiCr-Al2O3-SrSO4-Ag

RT-1000 oC, µ: 0.28-0.48, W:

[113]

Si3N4 ball; 5 N; 0.13 m/s;

Hot pressing sintering NiCr-BaMoO4

600 oC, µ: 0.26-0.30, W: 10-5 ~

[98]

Al2O3 ball; 5 N; 0.13 m/s; 800 oC,

NiCr-BaCr2O4[99]

µ: 0.27, W: 4.5 × 10-6 mm3/Nm

TE D

Hot pressing sintering

Inconel 718; 10 N; 1.0m/s;

Hot pressing sintering

RT-800 oC, µ: < 0.25; W: 1.9-13.1

[124]

EP

NiCrMoAl-12%Ag-10%CaF2/BaF2

× 10-5 mm3/Nm Si3N4 ball; 5 N; 1 m/s; RT-900 oC,

Hot pressing sintering

NiCrMoTiAl-12.5%Ag-(5-15%)CaF2/BaF2

AC C

10-6 mm3/Nm

[123]

µ: 0.23-0.31, W: 1.1-43.0 × 10-5 mm3/Nm Inconel 718; 5 N; 1 m/s; RT-900

Hot pressing sintering

o

NiCrMoTiAl-12.5%Ag-(5-15%)CaF2/BaF2

[123]

C, µ: 0.23-0.34, W: 0.8-39.4 × 10-5 mm3/Nm

Si3N4 ball; 5 N; 1 m/s; RT-700 oC, Hot pressing sintering NiCrMoTiAl-Ag-MoS2-CaF2 Hot pressing sintering

[120]

µ: 0.16-0.40, W: 1-29.4 × 10-5 mm3/Nm Si3N4 ball; 5 N; 0.8 m/s; Vacuum; 25

ACCEPTED MANUSCRIPT NiCrMoTiAl-12.5%Ag-5%CaF2/BaF2[118]

RT-800 oC, µ: < 0.25, W: ~ 10-5 mm3/Nm Si3N4 ball; 5 N; 0.785 m/s;

NiCrMoAl-12.5%Ag-(5-10%)CaF2/BaF2

[119]

Vacuum; RT-800 oC, µ: < 0.2; W: ~ 10-6 mm3/Nm

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Hot pressing sintering

Inconel 718 alloy disk; 2 N; 0.287 Hot pressing sintering

m/s; Vacuum; RT-700 oC, µ: <

NiMoAl-Cr2O3-Ag2Mo2O7 [112]

0.85-0.35; W: 90-2 × 10-5

SC

mm3/Nm

Inconel 718 alloy disk; 2 N; 0.287 m/s; Vacuum; RT-700 oC, µ: <

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Hot pressing sintering NiMoAl-Cr2O3-Ag2MoO4 [126]

0.94-0.26; W: 50-1 × 10-5 mm3/Nm

4.2.2 Intermetallic matrix high temperature solid-lubricating composites

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Since the strong internal order and mixed (metallic and covalent/ionic) bonding, intermetallic compounds often offer a compromise between ceramic and metallic properties when hardness and/or resistance to high temperatures is important enough

EP

to sacrifice some toughness and ease of processing [127-129]. Recently, investigations of intermetallic matrix materials have been initiated, consequently

AC C

leading to various novel high temperature solid-lubricating materials developments, such as nickel aluminum matrix [130-134], titanium aluminum matrix [135-138], ferric aluminum matrix [139], nickel silicon matrix and other high temperature solid-lubricating materials [140, 141]. Tribological properties of some intermetallic matrix solid-lubricating composites are displayed in Table 6. Ni3Al-based intermetallic alloys may be an excellent matrix for high temperature solid-lubricating composite owing to its high temperature strength, good oxidation resistance and corrosion resistance behavior. Recently, a series of Ni3Al matrix high temperature solid-lubricating composites were developed [15, 142-147]. The 26

ACCEPTED MANUSCRIPT solid-lubricating composites, which consist of Ni3Al matrix with solid lubricants (graphite, MoS2, hBN, Ag, Au, fluorides, inorganic salts etc.) and reinforcements (Cr, Mo, W, TiC, Al2O3, Cr2O3, Cr3C2, etc.), exhibit the low friction coefficient (µ < 0.35) and wear rate (10-4 ~ 10-6 mm3/Nm) at a wide temperature range from room

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temperature to 1000 °C. An optimized Ni3Al matrix high temperature solid-lubricating composite show satisfied mechanical and tribological properties, as shown in Figs. 3 and 4. The extensive investigations provide the valuable guide to design high temperature solid-lubricating composite, as shown in the following conclusions. The

SC

effect of solid lubricants on the tribological behavior shows that the combination of Ag and fluorides exhibits optimal synergetic lubricating action, whereas the effects of

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reinforcements on tribological and mechanical properties represent that the metal alloying phases are superior to ceramic phases. The low friction coefficient at a broad temperature range is mainly attributed to the integrated factors. (1) Tribo-chemistry. At high temperature ambient, fluorides react with alloying elements to produce inorganic salts with high temperature lubricous properties. (2) Tribo-transfer. At

TE D

various temperatures, the selected transfer films, which are the Ag-rich and fluoride-rich transfer film at low-moderate temperatures and the lubricating film containing inorganic salts at high temperatures, act a protective cover and reduce the

EP

direct contact of tribo-pair. (3) Tribo-glaze. At high temperatures, the glaze layer that is the complete and continuous oxide layer takes place on the worn surfaces, which is consisting of oxides and inorganic salts with lubricous properties. This glaze layer has

AC C

both wear-resistant and friction-reduced functions. (4) The synergistic action. The lubricating properties at a broad temperature range are mainly attributed to the synergistic action of Ag, fluorides and inorganic salts, which develop the various lubricous films with testing temperature under the action of friction force, lubricant diffusion and tribo-chemical reaction.

27

ACCEPTED MANUSCRIPT 1.0

1300

Friction coefficient Temperature

1000

1280

800 0.7 o

Temperature/ C

0.6

600

0.5 400

0.4 0.3

200 0.2 0.1

0

1240 1220 1200 1180 1160 300

200

100

0

0.0 0

20

40

60

80

100

120

20

800

900

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Friction coefficient

(b)

1260

0.8

Compressive strength/MPa

(a)

0.9

o

1000

Temperature/ C

Time/min

Fig. 3 Evolution of friction coefficient of a Ni3Al matrix high temperature solid-lubricating

composite with sliding time from room temperature to 1000 °C (10 N, 0.2 m/s, Si3N4 ball): (a);

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temperatures: (b)

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compressive strength of a Ni3Al matrix high temperature solid-lubricating composite at different

EP

Fig. 4 The lubricating behavior of Ni3Al matrix high temperature solid-lubricating composite at a broad temperature range

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The NiAl matrix composites with addition of CuO has a favorable friction coefficient of about 0.2 and excellent wear resistance with the magnitude of 10-6 mm3/Nm at high temperatures (800 °C and 1000 °C), while the NiAl-Cr-Mo-CaF2-Ag composite provides solid-lubricating properties of 0.2~0.4 at a broad temperature range between room temperature and 1000 °C [131, 132, 148, 149]. When AgVO3 works as solid lubricant, the NiAl-Mo-AgVO3 and NiAl-NbC-AgVO3 composites rubbing against Inconel 718 disk show satisfactory friction coefficient at 900 °C owing to the excellent lubricating effect of AgVO3 lubricants [130, 150, 151]. As for other solid lubricants, the NiAl-Ti3SiC2-MoS2 composite offers more favorable friction coefficient of 0.12~0.29 from room temperature to 800 °C compared to 28

ACCEPTED MANUSCRIPT NiAl-Ti3SiC2-WS2 composite and NiAl-PbO composite [134, 152]. TiAl intermetallics have acted as structural materials owing to their advantages of low density, high specific strength and specific modulus. To improve the poor wear and friction properties, in recent years, TiAl matrix solid-lubricating composites are concerned widely and developed, such as TiAl-Ag composite, TiAl-Ag-Ti3SiC2 TiAl-graphene-Ti3SiC2

composite,

TiAl-Ag-TiB2

composite,

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composite,

TiAl-Ag-BaF2/CaF2-Ti3SiC2 composite, TiAl-MoS2-Ti3SiC2 composite, and so on [135-137, 153-161]. The results show that the TiAl-Ag composite has a relatively favorable friction coefficient of 0.25-0.21 and wear rate of 10-4 mm3/Nm from room

SC

temperature to 900 °C [162].

Other intermetallic matrix high temperature solid-lubricating composites were also investigated. Fe3Al-Ba0.25Sr0.75SO4 solid-lubricating composites exhibits low

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friction coefficient of 0.19-0.29 and wear rate on the order of 10-5 mm3/Nm at 600~800 °C [139]. The improvement of friction and wear properties at high temperatures can be attributed to the lubricating film of Ba0.25Sr0.75SO4 formed on the frictional surfaces.

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Table 6 Tribological properties of intermetallic matrix solid-lubricating composites

29

ACCEPTED MANUSCRIPT Materials

Synthetic method and composite

Tested conditions and

composition

tribological results

Hot pressing sintering

Si3N4 ball;10 N; 0.2 m/s;

Ni3Al-Cr/Mo/W-Ag-CaF2/BaF2[15,

RT-1000 oC, µ: < 0.35, W:

142-146]

10-6-10-4 mm3/Nm Si3N4 ball; 20 N; 0.2 m/s;

Hot pressing sintering

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Ni3Al-Ag-BaMoO4

RT-800 oC, µ: 0.29-0.38, W:

[147]

10-5-10-4 mm3/Nm

Si3N4 ball; 20 N; 0.2 m/s;

Hot pressing sintering Ni3Al-Ag-BaCrO4

Ni3Al matrix

RT-800 oC, µ: 0.28-0.35, W:

[147]

10-5-10-4 mm3/Nm

composites

Spark plasma sintering

RT-800 oC, µ: 0.25-0.52, W: ~10-4 mm3/Nm

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Ni3Al-WS2-Ag-hBN[133]

Si3N4 ball; 10 N; 0.234 m/s;

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solid-lubricating

Spark plasma sintering Ni3Al-Ti3SiC2-Graphene

[163]

Spark plasma sintering

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Ni3Al-Ti3SiC2-WS2

[164]

Hot pressing sintering NiAl-Cr-Mo-ZnO[132]

Hot pressing sintering

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EP

NiAl-Cr-Mo-CuO[132, 148]

NiAl matrix

Hot pressing sintering

NiAl-Cr-Mo-CaF2

[131, 149]

Hot pressing sintering NiAl-Cr-Mo-CaF2-Ag

RT-750 oC, µ: 0.26-0.57, W: 3.1-6.5 × 10-6 mm3/Nm Si3N4 ball; 10 N; 0.2 m/s;

RT-800 oC, µ: 0.18-0.39, W: 1.5-3.7 × 10-5 mm3/Nm Si3N4 ball; 10 N; 0.2 m/s; 800-1000 oC, µ: ~0.35, W: 10-6-10-5 mm3/Nm Si3N4 ball; 10 N; 0.2 m/s; 800-1000 oC, µ: ~0.2, W: 10-6-10-5 mm3/Nm Si3N4 ball; 10 N; 0.2 m/s; 800-1000 oC, µ: ~0.2, W: 10-6-10-5 mm3/Nm

solid-lubricating composites

Si3N4 ball; 12 N; 0.2 m/s;

[149]

Si3N4 ball; 10 N; 0.2 m/s; RT-1000 oC, µ: 0.2-0.4, W: 10-5-10-4 mm3/Nm

Hot pressing sintering

RT~900 oC, µ: 0.25-0.6, W:

NiAl-NbC-AgVO3[130]

10-5~10-4 mm3/Nm

Hot pressing sintering NiAl-Mo-AgVO3

[150, 151]

Spark plasma sintering

718 pin; 2 N; 0.287 m/s; RT-900 oC, µ: 0.1-0.3, W: 10-5-10-4 mm3/Nm Si3N4 ball; 10 N; 0.2 m/s; 30

ACCEPTED MANUSCRIPT NiAl-Ti3SiC2-MoS2 [152]

RT-800 oC, µ: 0.12-0.29, W: 4.1-6.0 × 10-5 mm3/Nm Si3N4 ball; 10 N; 0.2 m/s;

Spark plasma sintering NiAl-Ti3SiC2-WS2

RT-800 oC, µ: 0.26-0.60, W:

[152]

3.5-6.5 × 10-5 mm3/Nm Si3N4 ball; 10 N; 0.2 m/s;

Spark plasma sintering

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NiAl-PbO

RT-800 oC, µ: 0.10-0.50, W:

[152]

4.1-9.2 × 10-5 mm3/Nm

Si3N4 ball; 12 N; 0.8 m/s;

Spark plasma sintering

RT-900 oC, µ: 0.25-0.21, W:

TiAl-Ag [162]

2.4-2.8 × 10-4 mm3/Nm

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Si3N4 ball; 10 N; 0.20 m/s;

Spark plasma sintering

TiAl-Ag-Ti3SiC2

TiAl matrix solid-lubricating composites

RT-600 oC, µ: 0.32-0.43, W:

[136]

Spark plasma sintering

1.87-2.0 × 10-4 mm3/Nm Si3N4 ball; 10 N; 0.234 m/s;

Spark plasma sintering

EP

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[130]

Hot pressing sintering

Fe3Al-Ba0.25Sr0.75SO4

1.0-2.0 × 10-4 mm3/Nm

RT-600 oC, µ: 0.19-0.61, W:

TiAl-Ag-V2O5 [160]

Fe3Al matrix

Si3N4 ball; 12 N; 0.3 m/s;

Si3N4 ball; 12 N; 0.3 m/s;

Spark plasma sintering

TiAl-Ag-BaF2/CaF2-Ti3SiC2

1.23-4.13 × 10-4 mm3/Nm

RT-600 oC, µ: 0.28-0.36, W:

[157]

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TiAl-Ag-TiB2

composite

0.87-2.87 × 10-4 mm3/Nm

Si3N4 ball; 10 N; 0.234 m/s;

Spark plasma sintering

solid-lubricating

RT-800 oC, µ: 0.34-0.56, W:

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TiAl-graphene-Ti3SiC2 [165]

[118]

RT-600 oC, µ: 0.33-0.45, W: 3.25-4.07 × 10-4 mm3/Nm Si3N4 ball; 10 N; 0.01 m/s; 600-800 oC, µ: 0.19-0.29, W: ~10-5 mm3/Nm

4.2.3 Ceramic matrix high temperature solid-lubricating composites In order to extend the permissible operating temperature upto 1000 °C, ceramic materials may be the sole and serious candidates for controlling friction and wear, and requiring corrosion/oxidation resistance. Advanced structural ceramics have high hardness between 15 GPa and 30 GPa at room temperature and such high hardness is maintained up to high temperature. Hence, advanced structural ceramics are expected 31

ACCEPTED MANUSCRIPT to be suitable for tribo-systems at elevated temperatures. However, the high hardness and low adhesive junction of ceramics does not exhibit beneficial effect for high wear resistance and low friction coefficient at elevated temperatures [166]. It is a result of the low toughness leading to microfracture at contact surfaces during sliding process,

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which reduces wear resistance and increases plowing effect. A major challenge in advanced structural ceramics is to develop long-lifetime and reproducible tribo-components for use in mechanical systems that involve high loads, velocities and temperatures. A feasible solution is to develop ceramic matrix high temperature

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solid-lubricating composites, which have gained extensive attentions among academics in recent years. Table 7 shows tribological properties of some ceramic

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matrix solid-lubricating composites.

Yttrium-stabilized tetragonal zirconia polycrystalline ceramic reveals high fracture strength, which responds through a phase transformation mechanism of tetragonal to monoclinic symmetry. Consequently, zirconia ceramic is a potential candidate matrix for high temperature solid-lubricating composite. The match of solid

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lubricants (e.g., graphite, MoS2, BaF2, CaF2, Ag, Ag2O, Cu2O, CuO, BaCrO4, BaSO4, SrSO4 and CaSiO3) for zirconia ceramic are investigated intensively to evaluate their potentials as effective solid lubricants at a broad temperature range [167-176]. It was

EP

found that CuO provides ZrO2 with low friction coefficient of 0.18-0.3 from 700 °C to 1000 °C [169], additionally, SrSO4 added ZrO2 offers low friction coefficient of below 0.2 and wear rates in the order of 10-6 mm3/Nm under very low sliding speed

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from room temperature to 800 °C [171]. The studies on the composite solid lubricants display the distinct tribological behavior at a wide temperature range. The combination of graphite and CaF2 reveals that graphite can provide low-temperature lubrication but it is feeble to improve tribological behavior of ZrO2 ceramic at 400 °C and 600 °C. As for ZrO2-Ag-CaF2 composite, a little content of Ag cannot provide lubrication for ZrO2 ceramic at low temperatures, while ZrO2 matrix composite containing large content of 35 wt% Ag can render a relatively low friction coefficient of about 0.4 from room temperature to 800 °C. With respect to the combination of MoS2 and CaF2, ZrO2-MoS2-CaF2 composite can offer satisfactory microhardness 32

ACCEPTED MANUSCRIPT (HV 824±90) and fracture toughness (6.5±1.4 MPa m1/2), and exhibit favorable friction coefficient of 0.25~0.40 from room temperature to 1000 °C owing to the synergistic action of MoS2, CaF2 and CaMoO4 formed on the worn surface by tribo-chemical reaction [167, 168]. Furthermore, the effect of tribo-pair on friction

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and wear properties of ZrO2-MoS2-CaF2 composite shows that ZrO2 matrix composite has a low friction coefficient in entire temperature range when sliding against Al2O3 ceramic, exhibiting high friction coefficient at moderate temperature when coupled with SiC and Si3N4 ceramics. The reason is that the hard SiOx particles generated

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from oxidation of SiC and Si3N4 ceramics destroy the lubricating CaF2 film on the worn surface at 600 °C.

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Alumina matrix high temperature solid-lubricating composite is a promising material because of its excellent chemical stability and low density. Al2O3-Ag-CaF2 composites exhibit moderate friction coefficient (typically between 0.35 and 0.55) and wear rate at temperatures of 200 oC to 650 oC, which are dominated by a synergistic effect of Ag and CaF2 on friction surfaces [177, 178]. Additionally, Al2O3 composites

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containing barite-type structure sulfates shows friction coefficient of 0.42~0.20 and wear rate of 10-3-10-5 mm3/Nm from room temperature to 800 oC [179]. Al2O3 matrix composites with laminated-gradient structure exhibit satisfactory strength and

[180, 181].

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tribological properties compared with traditional Al2O3 solid-lubricating composites

SiC ceramics possess low density, high hard, excellent high temperature

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properties, which are greatly suitable to be used in heat engine engineering and mechanical seal industry [182-196]. At present, many studies in this field mainly focus on wear resistance or water lubrication of SiC ceramic materials. Until now, there are still no satisfactory SiC matrix composites with favorable tribological performance over a wide temperature range from room temperature to 1000 oC. Recently, SiC matrix composites with addition of Mo and CaF2 exhibit favorable solid lubricity and wear resistance when mating with SiC ceramic. Especially at 1000 oC, it has the lowest friction coefficient of 0.17 and a favorable wear rate of 4.08 × 10-6 mm3/Nm [197]. To improve the lubricating properties at low temperature, further 33

ACCEPTED MANUSCRIPT studies reveal that the SiC-Mo-CaF2-MoS2 composite coupled with Al2O3 ceramic exhibits desirable friction coefficient of 0.14-0.41 and wear rate of 2.35 × 10-6 - 2.52 × 10-5 mm3/Nm from room temperature to 1000

o

C, whilst presenting poor

tribological properties at high temperatures of 600-1000 oC when sliding against SiC

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ceramic. This could be ascribed to the development of the completely lubricating layer on the worn surface at elevated temperatures.

Si3N4, as a great potential of high performance structural ceramic, has been widely used as ball bearings and engine parts owing to their high hardness, good

SC

corrosion resistance, good shock stability and excellent wear resistance. However, self-mated Si3N4 under unlubricated condition exhibits high friction coefficient and

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poor wear rate, especially at high temperature. In this case, the lubrication of Si3N4 ceramic has become the key issue. Currently, the solid-lubricating Si3N4-based composites toughened by in-situ formation of silver shows that nano-scale silver particles located at the grain boundaries effectively improve the fracture toughness of Si3N4, which results from the plastic deformation and crack bridging of silver [198].

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Furthermore, the Si3N4/Ag composite offers a relatively low friction coefficient of 0.3~0.6 and wear rate of 10-6-10-4 mm3/Nm from room temperature to 600 °C compared with pure Si3N4 ceramic, resulting from the lubrication and toughening of

EP

silver [199]. It should be noted that the in-situ silver formation method provide a guidance for the solution of agglomeration and outflow of low-melting-point metal in metal-ceramic materials during sintering process [198].

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The β-Sialon ceramics with hexagonal crystalline structure are derived from

β-Si3N4, by the equivalent substitution of Si-N by Al-O, have been recognized as the candidate for application in engineering systems due to their desirable mechanical properties,

excellent

thermal

shock

resistance,

good

chemical

stability,

high-temperature oxidation resistance, etc. However, less attention has been paid to tribological behavior of β-Sialons at elevated temperature. At 25 °C, β-Sialon has a preferable tribological property due to the excellent mechanical properties, but its tribological property of β-Sialon dramatically degenerate with increasing temperature [200]. The doping copper powder or copper coated graphite can significantly enhance 34

ACCEPTED MANUSCRIPT tribological behavior of Sialon matrix composites [201, 202]. The results showed that the tribological performances of Sialon composites can be significantly enhanced by doping copper powder at 800 °C and 900 °C, whilst the composites do not exhibit the lubricating effect at 25 °C and 600 °C [201]. Although the Sialon ceramic matrix

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composite with copper coated graphite can achieve the solid-lubrication at a wide temperature range, the mechanical properties have a severe degeneration [202]. In addition, high-performance TiN reinforced Sialon matrix composites with a good combination of excellent toughness and tribological properties at a wide temperature

SC

range were successfully prepared [203]. The fracture toughness of the composite with 10 wt% TiN reaches the maximum value of 6.2 MPa·m1/2 and 4.4 MPa·m1/2 at 25 °C

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and 800 °C, respectively. Moreover, as for the composite with addition of 30 wt% TiN at 200-800 °C, the friction coefficient reduces to 0.46-0.60, and the wear rate decreases about 15-50 times as compared to the Sialon without TiN. Boride ceramics are popular lubricating ceramics due to the formation of self-lubricating boric acid or boron oxide [204-212]. Of these, AlMgB14 is a novel

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superhard material after diamond, c-BN etc. It has orthorhombic structure with four icosahedral B12 units per unit cell, which is quite different from the traditional superhard materials that they have simple and high-symmetry crystal structure. The hardness is about 32-35 GPa and can be improved by 30-44% after incorporating TiB2,

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Si etc [204, 213]. Besides, it has a lower density of 2.6 g/cm3, a comparable thermal expansion with steel and Ti of 9 × 10-6 K-1 and excellent chemical inertness [204, 206].

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Moreover, the ternary ceramic and its composites exhibit outstanding self-lubricating and wear-resistant properties at room temperature and even high temperatures [214, 215]. In light of this, AlMgB14 is a good candidate using as the high temperature solid-lubricating material. The novel Mn+1AXn phase materials do not exhibit the anticipated solid-lubricating performance at high temperatures, though they possess a layered structure like graphite. Although there is the debate on their intrinsically solid-lubricating behavior, they could be appropriate candidate for high temperature solid-lubricating matrix due to the combination of metals and ceramics properties 35

ACCEPTED MANUSCRIPT [216-226]. In order to lubricate Mn+1AXn phases, many efforts have been made in recent years. Among them, Mn+1AXn matrix composites employed Ag as solid lubricant are the promising materials for high temperature tribological applications

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[227-231].

Table 7 Tribological properties of ceramic matrix solid-lubricating composites Synthetic method and coating

Tested conditions and tribological

composition

results

Materials

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Al2O3 ball; 20-800 °C; Spark plasma sintering

µ: 0.38-0.55;

W: 1.44~5.35×10-5 mm3/Nm

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ZrO2(Y2O3)-BaCrO4 [172] Spark plasma sintering

20~800 °C;

ZrO2(Y2O3)-Ag-CaF2 [174]

µ: 0.4-0.5

Al2O3 ball; 5 N; 1.0 m/s;

Spark plasma sintering

RT-800 oC, µ: 0.36-0.50;

composites

W: 1.67-3.55 × 10-6 mm3/Nm

Spark plasma sintering

Al2O3 ball; 5 N; RT-800 oC, µ < 0.2; W:

ZrO2(Y2O3)-Al2O3-SrSO4 [171]

1.05-2.28 × 10-6 mm3/Nm

Hot pressed sintering

EP

ZrO2 matrix

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ZrO2(Y2O3)-Au-CaF2[173]

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ZrO2(Y2O3)-Mo-BaF2/CaF2

RT-1000 oC, µ: 0.28-0.45; [176]

1000 oC, W: 9.46 × 10-5 mm3/Nm Al2O3 ball; 10 N; 0.2 m/s;

Hot pressed sintering ZrO2-MoS2-CaF2

SiC ball; 10 N; 0.2 m/s;

RT-1000 oC, µ: 0.2-0.4;

[168]

W: ~10-5 mm3/Nm

Hot pressed sintering

Al2O3 ball; 10 N; 0.2 m/s;

ZrO2(Y2O3)-CaF2-Mo-graphite[170]

RT-1000 oC, µ: 0.24-0.64; SiC ball; 10 N; 0.2 m/s;

Hot pressed sintering ZrO2(Y2O3)-Mo-CuO

700-1000 oC, µ: 0.18-0.3;

[169]

W: ~10-4 mm3/Nm 36

ACCEPTED MANUSCRIPT Spark plasma sintering

Al2O3 ball; 4.9 N;

Al2O3-BaSO4-Ag[179]

RT-800 oC, µ: 0.2-0.4;

Al2O3 matrix Al2O3 pin; 10 N; 0.168 m/s; Hot pressed sintering Al2O3-Ag-CaF2

20-800 oC, µ: 0.35-0.55;

[177, 178]

W < 3 × 10-5 mm3/Nm

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composites

SiC ball; 5 N; 0.1 m/s; Hot pressed sintering SiC-Mo-CaF2

[197]

SiC matrix composites

RT, µ: 0.3; 800 oC, µ: 0.2,

1000 oC, µ: 0.17; W: ~10-6 mm3/Nm

RT-1000 oC, µ: 0.14-0.41; W: 2.35 × 10-6 - 2.52 × 10-5 mm3/Nm

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SiC-Mo-CaF2-MoS2

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Al2O3 ball; 5 N; 0.1 m/s;

Hot pressed sintering

Si3N4 ball; 10 N; 0.13 m/s;

Si3N4 matrix

Spark plasma sintering

composites

Si3N4-Ag

RT-600 oC, µ: 0.3-0.6; W: 10-4-10-6

[199]

mm3/Nm

Si3N4 ball; 5 N; 0.10 m/s;

Sialon matrix

Si4Al2O2N6-TiN [203]

800 oC, µ: 0.54; W: 3.7 × 10-5 mm3/Nm

composites

Spark plasma sintering

Si3N4 ball; 5 N; 0.10 m/s;

Si4Al2O2N6-Cu [202]

600 oC, µ: 0.50; W: 5.4 × 10-5 mm3/Nm

Hot isostatically pressing

Inc718 disc; 3-18 N; 1 m/s; RT-550 oC;

Ta2AlC-Ag[227]

µ: 0.29-0.6; W < 5 × 10-5 mm3/Nm

Hot isostatically pressing

Al2O3 disc; 3-18 N; 1 m/s; RT-550 oC;

Ta2AlC-Ag[227]

µ: 0.39-0.45; W < 3-60 × 10-5 mm3/Nm

Hot isostatically pressing

Inc718 disc; 3-18 N; 1 m/s; RT-550 oC;

Cr2AlC-Ag[227]

µ: 0.41-0.8; W < 7-10 × 10-5 mm3/Nm

Hot isostatically pressing

Al2O3 disc; 3-18 N; 1 m/s; RT; µ: 0.55;

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EP

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Spark plasma sintering

MAX matrix composites

Cr2AlC-Ag

[227]

W < 7 × 10-5 mm3/Nm

5. Applications of high temperature solid-lubricating materials With the development of modern industry, one hand is that more machinery 37

ACCEPTED MANUSCRIPT components of high-end equipment service in high temperature environment; another hand is that frictional surface temperature increases significantly since the miniaturization, lightweight, high performance and high efficiency of mechanical system lead to increase in energy density on frictional surface. There is no doubt that

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it is necessary to solve the problems of friction and wear by applying high temperature tribology theory. Industrial applications involving in high temperature lubrication include bearing, seal, machining tool, current collector, forming mould, and so on [232-235].

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5.1 Bearing

High temperature solid lubricating bearing is a novel oil-free lubrication

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technology, which can simplify the lubrication and cooling system and consequently enhance energy transmission efficiencies [22, 26, 28, 217]. At present, high temperature bearing has been applied in many fields, such as high temperature light load swing bearing of shuttle, air foil bearings, high temperature rolling bearing of gas turbine engine, rudder bearing of supersonic aircraft, bearing of adiabatic diesel

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engine, etc. As for foil bearings, when working routinely over a wide temperature range from cryogenic to over 650 °C, they require solid lubrication to minimize wear and reduce friction during start-up and shut-down periods and also during momentary

EP

bearing over loads. PS304 coating, as the shaft coating, is successfully applied to foil gas bearings, which lives well in excess of 100,000 start/stop cycles [236, 237]. 5.2 Seal

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The rising of energy efficiency and working temperature of the mechanical

system requires heat sealing materials to possess favorable high temperature tribological properties, such as seal components of the nuclear power pump, piston rings of Stirling engine. The piston ring/cylinder in the Stirling automotive engine works at 600 to 1000 °C near the top of the cylinder walls and in a strongly reducing hydrogen atmosphere [238]. After evaluated in a Stirling engine test, PS200 coating has shown promise for use as a piston ring material. 5.3 Machining tool High temperature is one of important failure factors for machining tool. During 38

ACCEPTED MANUSCRIPT high-speed cutting (50-300 m/min), cutting tool overcomes severe deformation of the work piece and high contact stress (even upto 1 GPa) of cutting edge. As a consequence, surface temperature at the contact area can reach 1000 oC or above. Most recent advancements in tool coatings are targeting solid-lubricating properties in

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the cutting zone to minimize frictional heat and reduce energy consumption during machining operations [239-241]. On the basis of tribo-chemical reaction, solid-lubricating tool materials like MoN-Cu nitride coating and Al2O3-TiB2 bulk is one of the ongoing efforts in cutting tool field [239, 242].

SC

5.4 Forming mould

Material forming mould that services in high temperature condition for a long

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term are prone to wear and fatigue damage under friction stress, resulting in failure of mould. The improvement of high temperature lubricating and wear properties for mould material is an importance to prolong the service life and reduce the production cost. During most hot extrusion process, the strong extrusion force (1000-2500 MPa) and high temperature (800-1200 oC) have significant influence on the service life of

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dies. An approach to reduce friction force on contact surface is an effective demoulding technology. Due to the short run time during material forming, a feasible solution is to allow the lubricating powders to coat on the contact surface, such as

EP

glass lubricant (major components: SiO2 and B2O3) working at 500-2200 oC. When glass lubricant contacts with high temperature billet, it can form a fluid film between tool and billet, and then separates the two contact surfaces. This fluid film has the

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effect of lubrication and heat insulation. 5.5 Current collector

Tribology of electrical contacts involves the coupling action between frictional

contact and electric contact [243-246]. Current collector is the main collecting electricity component to obtain electric energy, such as pantograph contact strip for electric railways, slip-roll ring for electric transmission in space, and so on. With rapid development of high-speed train, pantograph contact strip undergoes more harsh service conditions (rated current: 1000 A, rated voltage: 25 KV, sliding speed: 300 km/h, applied load: 70 N, wear loss: 1 mm/10000 km). Moreover, arc discharge due to 39

ACCEPTED MANUSCRIPT break of contact between the contact strip and contact wire inevitably occurs. Current collector with high-power and long-life performance endures arc discharge attack and high temperature mechanical wear. The development of current collector material with excellent electrical conductivity and lubricating properties has always been the

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focus of tribology of electric contacts, such as noble metal slip-roll ring for electric transmission in space, metallized carbon contact strip for electric railways. 6. Trends of high temperature solid-lubricating materials

Although considerable progress has been made in investigating lubricating

SC

mechanism and exploring high temperature solid-lubricating materials, high temperature lubrication is confronted with many practical challenges with the

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development of modern industry. At present, high temperature tribology still lacks systematically fundamental theory, simply drawing on the experience of solid lubrication theory. Compared to liquid lubrication, it is imperative to further minimize wear and reduce friction of high temperature solid-lubricating material. To fulfill extensive engineering applications, high temperature solid-lubricating material should

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be designed according to the general criterion: a friction coefficient < 0.2, a wear rate < 10-6 mm3/Nm, and a wide temperature range from low temperature to high temperature (as shown in Figs. 5 and 6). To achieve the above aims, there is still a

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need to perform the following systematic studies.

40

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ACCEPTED MANUSCRIPT

Fig. 5 The requirement of friction coefficient and wear rate for high temperature

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EP

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solid-lubricating material

Fig. 6 The allowable temperature range for various high temperature solid-lubricating materials

6.1 Solid lubrication Solid lubrication is still feasible and reliable high lubrication technology. More investigation should be focused on the synergistic effect of solid lubricants and the exploration of novel solid lubricant to achieve a wide-environment-range lubrication, 41

ACCEPTED MANUSCRIPT a super-high temperature lubrication, and a long-lifetime lubrication. 6.2 Smart lubricating material Currently,

more

efforts

are

addressed

to

develop

high

temperature

solid-lubricating material with single lubricity to meet the requirement of industry.

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Integrated equipment leads to the increase in material flow, information flow and energy flow, resulting in the emphases on the studies on the multifunctional lubricating material,

such as the structural/lubricating integrated

material,

anti-radiation lubricating material, conductive or insulation lubricating material, etc.

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Smart solid-lubricating material should be an important direction in the future, which has the abilities of self-diagnosis, self-repair, and self-adjust. Rough classification of

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high temperature solid-lubricating material is illustrated Fig. 7. At present, self-adaptive lubricating material, as an elementarily smart lubricating material, offers a favorable design idea and feasible technologic method. 6.3 Multidisciplinary cross connection

Aiming to the future trend of high temperature lubrication, we should strengthen

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on multidisciplinary cross connection involving materials science, chemistry, physics, biology, mechanical engineering. With the development of computer technology and information technology, materials genome initiative and three-dimensional printing

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technology bring about a good development opportunity for high temperature tribology [247-250]. Additionally, bionic design can also provide us with a new approach.

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In future, under the lead of multidisciplinary cross connection and advanced

manufacturing technology, high temperature solid-lubricating material make a great stride from self-adaptive materials to smart or intelligent lubricating materials.

42

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Acknowledgment

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Fig.7 Future trends of high temperature solid-lubricating material

This work was funded by the National Key Research and Development Program

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of China (2018YFB0703803) and the National Natural Science Foundation of China (51675510, 51675511 and 51775532).

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