Development of nanostructured slide coatings for automotive components

CHAPTER 18

Development of nanostructured slide coatings for automotive components Andreas Gebhard, Frank Haupert and Alois K. Schlarb

Contents 18.1 18.2 18.3 18.4 18.5

Introduction Mechanical properties of slide coatings Matrices for slide coatings Nanoparticle-filled slide coatings for automotive applications Conclusions References

439 441 445 448 454 455

Abstract Polymer based slide coatings have become indispensable as tribological layers of slide bearings. In modern automobiles they are used as tribological coatings for high-temperature applications in the engine compartment such as anti-friction coatings for piston skirts or in low-lubricant applications like journal bearings of diesel fuel injection pumps which often operate under boundary lubrication. Important structure–property relationships for the achievement of a high operating temperature and a high-compression loading capability are discussed in detail for two of the most important polymeric matrices: epoxy resins (EPs) and polyamide imide (PAI). The beneficial effect of nanoparticulate fillers on the tribology of slide coatings is demonstrated on the basis of three current applications from the automotive industry: slide coatings for piston skirts, polymer/metal-slide bearings for high-temperature bearing applications and for use under boundary lubrication.

18.1 Introduction One of the major drawbacks of the internal combustion engine is its dependence on limited fossil fuel and, coupled to this, its emission of greenhouse gases like carbondioxide (CO2 ) or nitric oxides (NOx ) as well as the emission of harmful particulates [1]. In 2004, the overall CO2 -emission in the U.S. was estimated to be 6.0 billion tons. Thirty percent of this, i.e. 1.8 billion tons, is caused by the burning of fossil fuels in transportation [2]. It has been estimated that a 10% reduction of mechanical losses would lead to a 1.5% reduction in fuel consumption [3] which would correspond to an annual saving of 27 million tons of CO2 in the U.S. alone. The annual benefit of the reduction of the internal combustion engine’s friction and wear to the U.S. economy are estimated to US$ 120 billion. Overall, the combined effect of the increasing environmental awareness of customers and of the more stringent global competition and governmental regulations on greenhouse gas emission has made the increase of fuel economy a highly important field of tribological research during the past decades [4–8]. In medium-sized passenger vehicles the major portion of energy loss occurs in the engine (41% of total energy consumption), followed by vehicle weight (29%) and rolling resistance (15%) [9, 10].

439

440

Tribology of Polymeric Nanocomposites

Overall, the engine accounts for 66% of the total frictional loss, most of which is caused by the sliding of the piston rings (19%) and the piston skirt (25%) against the cylinder wall and by the engine’s bearings (22%). In two of these tribological systems, namely slide bearings and piston skirt coatings, polymeric slide coatings constitute the state-of-the-art and effectively abate friction and increase wear resistance. Modern slide coatings consist of three basic components: a binder, solid lubricants and a solvent. Since they are basically varnishes with color pigments exchanged by friction and wear reducing components, they can be applied using standard procedures, e.g. spraying followed by heating in an oven [11, 12]. Table 18.1 shows a selection of the wide range of the commercially available slide coatings. As can be seen thermosets are the most popular polymeric binders for slide coatings but there are also coatings based on high-performance thermoplastics, such as polyamide imide (PAI). There is also a wide variety of functional fillers available, e.g. solid lubricants for reducing friction, reinforcing fibers for high mechanical strength or hard particles for abrasion resistance. Recently all of these fillers have become commercially available as nanoscale fillers which lead to significant research in the field of nanocomposites and especially in the field of nanocomposite tribology. In automotive applications nanocomposites are considered to be the key to significant reductions of friction, wear oil and fuel consumption as well as the prolongation of tribological components’ lifetimes, higher Table 18.1. Commercially available slide coatings (selection). Brand

Manufacturer Binder

Solid Solvent lubricant(s)

Key features Corrosion protection

Scotchkote- 3M Series DAG 213 Acheson Industries

EP

Unknown

EP

Graphite

DAG 154

Acheson Industries

Cellulose

Graphite

Molydag 250 SermaLube Everlube 9002

Acheson Industries SermaGard Everlube

Thermoset MoS2 , graphite Unknown Unknown EP MoS2

Klübertop 06-111

Klüber Lubrication

Organic

MoS2

Moly-Paul ITC-Bond

KS Paul

Resin

MoS2

Almasol 9200

Lubrication Engineers

Resin

Unknown

OKS 536

OKS

Unknown Graphite

AI-10

Solvay Adv. Polymers

Polyamide Unknown imide

Xylan 5250

Whitford Worldwide

Resin

∗

PTFE

Powder coating Unknown

Good adhesion, good lubrication at elevated temperatures, good wear protection Isopropanol Maximum operation temperature 400 ◦ C, for use in internal combustion engines, dries at room temperature Organic Maximum operation temperature 350 ◦ C, good wear protection Unknown – Water* High resistance to sliding and abrasive wear, highly resistant to chemicals, for high loading, water-based Unknown Maximum operation temperature 220 ◦ C, highly resistant to chemicals, loading >10 MPa Methylacetate, High corrosion and wear resistance, isopropanol maximum operation temperature 300 ◦ C, used for piston skirts. Unknown Operation temperature −73 ◦ C to 343 ◦ C, loading up to 670 MPa Water* Highly loadable, temperature resistant, water-based Unknown Maximum operation temperature 260 ◦ C, high chemical resistance (e.g. jet fuel, motor oil, diesel fuel) Water* Corrosion and wear protection, water-based

Contains small amounts of volatile organic compounds (VOC).

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permissible pv-products and higher operation temperatures. Furthermore, slide coatings based on nanocomposites offer the well known advantages of polymeric anti-friction coatings, namely increased corrosion resistance compared to metallic tribosystems, reliable long-term protection from fretting and galling, low cost, wide applicability and superior dry running capabilities, which is important in the case of oil lack or during cold starting.

18.2 Mechanical Properties of Slide Coatings In automotive applications some of the main requirements for slide coatings are high operation temperature, high mechanical properties, low friction and a high wear resistance. Once the slide coating is applied to its substrate and cured, all material properties are in principle influenced by both matrix and fillers (Fig. 18.1). Nevertheless, there are some properties that are predominantly determined by the matrix, e.g. thermal stability, damage tolerance, toughness and chemical resistance. However, there are properties which mainly depend on the fillers, such as strength, elastic modulus, wear resistance and coefficient of friction. To obtain high mechanical properties, fibers are the most important common fillers for slide coatings. They increase the coating’s load-carrying capacity, its maximum operation temperature and they reduce its thermal coefficient of expansion and thereby the thermal stress between the coating and the metallic substrate. The action of nano and micro-scale fillers on bulk materials is extensively reviewed in Chapter 3 by L. Chang et al. In general the improvement of the wear resistance and the lowering of the coefficient of friction of bulk polymeric composites by functional fillers also holds true for thin films used as slide coatings. Therefore this section focuses on the description of selected structure–property relationships that allow for the optimization of a coating’s operation temperature and compression loading capability by the careful selection of a proper matrix and optimum filler contents.

18.2.1 Operation temperature The temperature threshold up to which polymers can be used as matrices for coatings is limited by the thermal stability of the matrix itself, the retention of its mechanical properties and the deterioration of the composite’s tribological performance at elevated temperatures [13, 14]. As will be demonstrated in Section 18.4.2.1 nanosized anti-wear modifiers have proven to be especially useful in obtaining a high wear resistance and a low coefficient of friction at elevated temperatures. Property

Influenced by:

Matrix

Fillers

Elastic modulus Strength Toughness Damage tolerance Tribological behaviour Impact behaviour Corrosive behaviour Temperature resistance Chemical resistance Electrical properties Manufacturing

Fig. 18.1. Relative influence of matrix and fillers on selected properties of a slide coating.

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Tribology of Polymeric Nanocomposites Table 18.2. Residual mechanical properties at 150 ◦ C of a bisphenol A-resin cured with two different amine hardeners.

Tensile strength (MPa) Tensile modulus (GPa) Compressive strength (MPa) Compressive modulus (GPa) Flexural strength (MPa) Flexural modulus (GPa)

Bisphenol DGEBA/DDM

Bisphenol DGEBA/DDS

At 20 ◦ C

At 150 ◦ C

At 20 ◦ C

At 150 ◦ C

53 2.75 111 2.67 116 2.73

19 1.54 29 0.72 41 1.68

59 3.07 107 2.0 102 2.79

37 1.47 63 1.28 49 1.16

Source: Data from refs [16, 17].

Basically the retention of the composite’s mechanical properties at elevated temperatures can be facilitated by choosing thermoplastics with a high glass transition temperature or thermosets with a high softening temperature, respectively. While the matrix defines the basic thermal performance of the composite, the maximum operation temperature of polymers, i.e. of thermoplastics and thermosets, can be increased by the addition of functional fillers like glass or carbon fibers. While for thermoplastics the composite’s thermal resistance and the retention of its mechanical properties at elevated temperatures is defined by the matrix, a significant influence of the specific chemical nature of the resin/hardener combination is observed for thermosets. Table 18.2 demonstrates this by comparing the thermal retention of selected mechanical properties of a bisphenol A resin that has been cured with 4,4 -diamino diphenyl methane (DDM) and 4,4 -diamino diphenyl sulfone (DDS), respectively. The higher performance of DDS- over DDM-cured resins is attributed to a more rigid cross-linking due to the higher rigidity of the sulfone moiety compared to the methylene moiety [15]. The chemical structures of the resin and the hardener furthermore significantly influence the cured material’s resistance to thermal degradation. In contrast to the maximum operation temperature and the retention of mechanical properties, the thermal degradation of the matrix is hardly improved by functional fillers. While most fillers, such as hard ceramic particles, layered silicates, fibers, solid lubricants or carbon nanotubes are unaffected by temperatures of up to 400 ◦ C, polymeric matrices suffer severe thermal degradation. The main mechanisms therefore are chemical reactions which break up the polymer chains to form smaller and more stable molecules. One of the first unambiguous demonstrations of this effect was carried out for the diglycidyl ether of bisphenol A (DGEBA) that has been cured with DDM and with the acidic anhydride hardener methyl nadic anhydride (MNA) respectively [18]. In Thermogravimetric Analysis (TGA) under nitrogen atmosphere DGEBA/DDM does not exhibit any weight loss up to 300 ◦ C. For DGEBA/MNA however thermal degradation occurs in a two-step process with the first significant weight loss already taking place at only 150 ◦ C. This is caused by the decarboxylation of esters formed during the curing. This decarboxylation is furthermore considered to generate free radicals, which are highly reactive chemical species that damage the cured resin’s molecular structure in addition to the effect of the decarboxylation. Since amine hardeners form thermally more stable β-hydroxypropylamines instead of esters, resins cured with polyamines generally exhibit a higher thermal stability. Information concerning the chemical resistance of polymers is frequently given for standard conditions. Very often this means exposure at 20–25 ◦ C for some hours to a few days. When selecting a polymeric matrix for a high-temperature slide coating that will be exposed to chemicals, it must be kept in mind that the chemical resistance of polymers is frequently given for standard laboratory conditions. This usually means exposure at 20–25 ◦ C for a period of some hours to a few days. Certain forms of chemical attacks, such as oxidation or hydrolysis however become disproportionately severe with increasing temperature and/or exposure time. PAI for example is significantly impaired by hot

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water (see Section 18.3.1.1 for more information). This is especially problematic when PAI-based slide coatings are to be exposed to oil or fuel, since these two liquids may contain up to 5% of water during cold start conditions [4].

18.2.2 Compression loading capability Since slide coatings substitute metallic surfaces like the bronze layer in journal bearings, they need a high compressive strength and a high compression modulus. As can be seen from Fig. 18.1 the mechanical properties of a composite material mainly depend upon the action of the fillers. The compressive properties of polymeric composites are therefore most commonly improved by the addition of short fibers in contents between 10 and 30 vol.%. While clear relationships between the tensile properties of a fiber-reinforced composite and the fundamental tensile properties of the fibers themselves have been established, only very little is known about the analogous relationship for compression loading. This is mainly due to the lack of an experimental procedure that allows for the direct determination of the compressive properties of fibers with diameters of only a few micrometers. Therefore indirect methods like the subjecting of unidirectional reinforced polymers to macroscopic standard compression tests are used. From these the fibers’ fundamental compressive properties are then estimated by a rule of mixtures [19, 20]. For a detailed review of testing methods for the compressive properties of fibers see the review by Kumar [21]. The predominant failure modes of fiber-reinforced polymers under compression loading include fiber kinking, longitudinal splitting and pure compression or shear failure of the fiber [22]. Most of the currently available micro-buckling models [23, 24] assume that the kinking of the fiber is fostered by insufficient lateral support from the matrix. This lateral support originates from the shearing and compression of the matrix due to the radial expansion of an axially compressed fiber. This mechanism has been investigated experimentally by an indirect test method that subjects a composite to a compressive strain higher than the fibers ultimate strain. The fibers’ mechanical properties are then estimated from the mean fragment length of fibrillar-failed fibers. Using this method the compressive strengths of different polyacrylnitrile (PAN)- and pitch-based fibers in an epoxy resin (EP) matrix were studied [25]. Since the lateral support of the fibers depends upon the stiffness of the surrounding matrix, the temperature of the samples was increased from 20 ◦ C to 100 ◦ C in order to soften the matrix and to hence to reduce the fibers’ lateral support. For all fibers studied a steady decrease in the composite’s apparent compressive strength was found with increasing temperature. Since the real compressive strength of carbon fibers is considered to be constant in this temperature interval, the observed effect must be due to a change in the fiber-matrix interaction. One of these interactions is a thermal stress between the fiber and the surrounding matrix which acts perpendicular onto the fiber surface. This stress originates from the different coefficients of thermal expansion of the matrix and fiber during the curing of a resin or during the cooling down of a thermoplastic. For the investigated short carbon fiber reinforced epoxy resin (EP/SCF) systems a change in the composite’s microscopic failure mode was observed: while at temperatures below 100 ◦ C compressive failure of the fiber itself occurs, buckling is found to be the predominant failure mode at 100 ◦ C and above. The thermal stress due to different thermal expansion was estimated to be approximately 6 MPa at 20 ◦ C. At 100 ◦ C however this stress is relaxed to only approximately 0.01 MPa and at the same time the stiffness of the EP matrix is reduced to 1% of its corresponding stiffness at 20 ◦ C. Therefore the observed change of the failure mode is reasonably attributed to the loss of lateral support by thermal deterioration of the fiber/matrix-prestrain and of the matrix stiffness. The “real” fiber compressive strengths were obtained from the observed temperature dependence of the embedded fibers strength by extrapolation to zero thermal stress and zero elastic modulus of the matrix (Table 18.3). There are significant differences in the compressive properties of high strength (HT) and high modulus (HM) fibers. The precursor from which the fibers have been made also seems to play an important role since differences between fibers made from polyacrylnitrile (PAN) and pitch have been reported.

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Tribology of Polymeric Nanocomposites Table 18.3. Absolute and relative compressive strengths of selected short carbon fibers.

Tensile strength (GPa) Estimated compressive strength (GPa) Estimated compressive strength (% of tensile strength)

Torayca T-300 (High Strength PAN fiber)

Torayca M-40 (High Modulus PAN fiber)

Tonen HTX (High Strength Pitch fiber)

Tonen HMX (High Modulus Pitch fiber)

3.50 2.06

2.88 0.78

3.34 1.25

4.33 0.54

59

27

37

12

Source: Data from ref. [25].

Table 18.4. Absolute compressive strengths of selected polymeric fibers. Compressive strength (GPa) UHMWPE Kevlar PBO PBT Pitch CF S-2 Glass PIPD PAN CF

0.17 0.34–0.48 0.2–0.4 0.26–0.41 0.48 1.1 1.0–1.7 1.05–2.75

Source: Data from refs [22, 26].

While the compressive strength of the HT-PAN fiber Torayca T-300 amounts to nearly 60% of its tensile strength (3.5 GPa) the compressive strength of the HM-PAN fiber Torayca M-40 amounts to only 27% of its tensile strength (2.9 GPa). The same is true for fibers based on pitch, i.e. high strength fibers have higher absolute and relative compressive strengths than high modulus fibers. Overall, PAN-based fibers possess higher absolute compressive strengths than their pitch-based counterparts. Additionally their compressive strengths (related to their tensile strength) are significantly higher than for pitch-based fibers, which makes them first choice for reinforcing highly compression loaded slide coatings. Besides pure carbon fibers there are also fibers spun from polymers based on defined organic species. The most common ones are based on ultra high molecular weight polyethylene (UHMWPE), polyethylene- and polybutylene terephthalate (PET, PBT), organic aramides (Kevlar, Nomex) and polyphenylene benzobisoxazole (PBO). PBO currently yields fibers with the highest tensile modulus and the highest tensile strength of all polymeric fibers. Additionally, PBO is among the most thermally stable of all commercially available polymeric fibers. However, it exhibits a rather poor axial compressive strength of only 200–400 MPa, which corresponds to only 4–7% of its tensile strength [26]. The axial compressive strength being only a fraction of the corresponding tensile strength is a common flaw of most polymeric fibers as can be seen from Table 18.4, which compares the compressive strengths of several fiber types. Currently the polymeric fiber with the highest axial compressive strength is based on polydi(imidazo) pyridinylene phenylene (PIPD). This polymer forms strong intermolecular hydrogen bonds in two dimensions which translate into a fiber compressive strength of 1.0–1.7 GPa which is the highest value found for polymeric fibers to this date. Nevertheless, the compressive strength of PAN-based carbon fibers is unmatched by any other fiber type and concerning the maximum achievable improvement it is therefore the first choice for enhancing the compressive strength of slide coatings. However, carbon fibers with a high axial compressive strength tend to have a lower tensile modulus [27]. For a detailed review on the compressive properties of carbon and polymeric fibers see the reviews of Kumar [21, 26–28] and the references given therein.

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Table 18.5. Comparison of the compressive properties and the impact strengths of selected high temperature thermoplastics.

Torlon 4203 PAI (polyamideimide) Tempalux PEI (polyetherimide) Techtron PPS (polyphenylene sulfide) Ketron PEEK (polyetheretherketone)

Compressive strength (MPa)

Compressive modulus (GPa)

Impact strength (Izod, notched) (J/cm)

166 151 148 138

3.3 3.3 3.0 3.5

1.06 0.53 0.23 0.53

Overall, the matrix plays an important role in achieving high compressive loading composites by fiber reinforcement. While it is true that the fiber’s compressive strength sets the upper limit that a composite’s strength can reach, a proper matrix that is able to laterally support the fiber is required in order to prevent early failure due to micro-buckling. Since the shear modulus of the matrix determines the maximum radial load that can be transferred from the fiber to the matrix at any given axial fiber strain, a matrix with a high modulus of elasticity is favorable. According to the combined influence of fiber stiffness and lateral support by the matrix, buckling occurs for low modulus fibers (e.g. aramid fibers) and low modulus matrices (E < 3 GPa), whereas stiff PAN- or pitch-based fibers (E > 350 GPa) rather exhibit compressive failure of the fiber itself. Again, this is even more important at elevated temperatures, since the general decrease of the polymer’s mechanical properties and the relaxation of residual thermal stresses between the matrix and the fiber reduce the fiber’s lateral support by the matrix. Compressive failure based on micro-buckling can be reduced by higher matrix shear strength and yield strength, by increasing the fiber-matrix adhesion and by reducing fiber waviness and defect content [22, 29]. While fiber waviness is rather a problem of laminates, an optimization of the fiber orientation into loading direction should be aimed at for short fiber-reinforced composites so that a bigger fiber fraction is loaded axially. Furthermore can micro-buckling be prevented increasing the resin’s shear modulus for increasing the lateral load that is transferred to the matrix at a given strain or by introducing a prestrain between the fiber and the matrix during fabrication. The enhancement of the fiber-matrix bonding strength can reduce fiber slippage and increases the transferable axial load. If pure compressive failure of the fibers is the microscopic reason for macroscopic failure the only way to achieve better loading capability is to replace the current fiber with a stiffer fiber.

18.3 Matrices for Slide Coatings When selecting a polymeric binder for a slide coating the first choice is whether to use a thermoset or a thermoplastic, of which each possesses inherent advantages and disadvantages. Generally, thermosets have higher mechanical strength and stiffness, but at the same time they are more brittle than thermoplastics. The latter are the preferred option for dynamically loaded parts since they have a better damage tolerance, toughness and dampening, which greatly helps in reducing noise emission. This section focuses on the detailed discussion of EPs since they are the most common matrices for commercial slide coatings and on PAI since it is a high-performance thermoplastic that is also a very frequently used commercial slide coatings, especially for piston skirts [30, 31].

18.3.1 Polyamide imide (PAI) Although PAI is an amorphous thermoplastic it exhibits high mechanical strength and stiffness, high creep and thermal resistance as well as excellent tribological properties. Neat PAIs compressive strength is 166 MPa which is higher than the values found for other high performance thermoplastics (Table 18.5). Its compressive modulus of 3.3 GPa is sufficiently high to effectively prevent fiber buckling. PAI can therefore be efficiently reinforced with short fibers. Upon the addition of 30% short

446

Tribology of Polymeric Nanocomposites Table 18.6. Mechanical and thermal properties of selected PAI-composites [32, 33].

Composition (wt%)

Density (g/cm3 ) Tensile strength (MPa)

Tensile modulus (GPa) Strain at break (%)

Flexural strength (MPa)

Flexural modulus (GPa)

Compressive strength (MPa) Compressive modulus (GPa) Impact strength (Izod) (J/m) Water absorption (%) Shrinkage (%)

SCF SGF Graphite PTFE TiO2

0 0 20 3 0

0 0 12 3 0

0 30 0 0 0

30 0 0 0 0

0 0 0 0.5 3

0 0 12 8 0

−196 ◦ C 23 ◦ C 135 ◦ C 232 ◦ C 23 ◦ C −196 ◦ C 23 ◦ C 135 ◦ C 232 ◦ C −196 ◦ C 23 ◦ C 135 ◦ C 232 ◦ C −196 ◦ C 23 ◦ C 135 ◦ C 232 ◦ C 23 ◦ C 23 ◦ C notched un-notched (24 h)

1.51 130 131 116 56 7.8 3 7 15 17 203 212 157 111 9.6 7.3 5.6 5.1 124 4.0 85 250 0.33 0.4

1.46 – 164 113 73 6.6 – 7 20 17 – 219 165 113 – 6.9 5.5 4.5 166 5.3 60 405 0.28 0.5

1.61 204 205 160 113 10.8 4 7 15 12 381 338 251 184 14.1 11.7 10.7 9.9 264 7.9 80 505 0.24 0.2

1.48 158 203 158 108 22.3 3 6 14 11 315 355 263 177 24.6 16.5 15.6 13.1 254 9.9 50 340 0.26 <0.2

1.42 218 192 117 66 4.9 6 15 21 22 287 244 174 120 7.9 5.0 3.9 3.6 221 4.0 140 1070 0.33 0.7

1.50 – 123 104 54 6.0 – 9 21 15 – 189 144 100 – 6.3 4.4 4.3 130 – 69 – 0.17 –

carbon fibers the compressive strength is increased to 256 MPa. Another advantage of PAI over other thermoplastics is its high impact strength. These excellent mechanical properties are attributed to PAIs rigid polymer backbone and to pronounced non-covalent interaction between the separate polymer chains. Although it is an amorphous thermoplastic, PAI requires a post cure after being applied to surfaces. During this, its molecular weight is increased by covalent cross linking resulting in an increase of the chemical and thermal stability. If correctly tempered, PAI exhibits an excellent temperature resistance and residual mechanical properties at high temperatures (Table 18.6). The neat PAI matrix is chemically degraded by hot steam, high pH-alkalines such as a 30% aqueous solution of sodium hydroxide or by strong organic acids such as benzene sulfonic acid or formic acid (88%). The mechanism of the chemical attack is considered to be the disruption of hydrogen bonding and/or the chemical decomposition of the amide bond in the polymer chain backbone. Otherwise, the neat PAI matrix exhibits an excellent chemical resistance against a wide range of weak acids and caustics, aqueous saline solutions, alcohols, aldehydes and ketones, chlorinated organic compounds, esters, ethers, unpolar hydrocarbons, nitriles, nitro compounds and, most important concerning automotive applications, PAI is highly resistant against diesel fuel, gasoline, hydraulic oil and motor oil [33]. The glass transition temperature of neat PAI is 285 ◦ C, which constitutes its thermal limit since PAI is completely amorphous. Without mechanical loading, no thermal degradation of the neat PAI matrix is observed below 300 ◦ C. At 250 ◦ C it takes several thousand hours in order to deteriorate PAIs tensile strength noticeably [34]. At a loading of 1.82 MPa, PAI filled with 3 wt% titanium dioxide (TiO2 ) and 0.5 wt% polytetrafluorethylene (PTFE) exhibits a heat deflection temperature of 274 ◦ C which among the highest known for polymeric composites. Owing to its excellent thermal resistance and its very

Specific wear rate (106 mm3/N m)

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447

pv  0.345 MPa.m/s pv  1.380 MPa.m/s pv  1.725 MPa.m/s

0.8 0.6 0.4 0.2 0.0

12/3 20/3 12/8 Graphite content (%)/PTFE content (%)

Fig. 18.2. Specific wear rates of the commercially available bearing grade PAI materials (data from ref. [33]).

good mechanical properties up to 230 ◦ C PAI is highly favorable for the design of tribological coatings for use in the engine compartment of a vehicle or even in an internal combustion engine. Besides its very high temperature resistance, PAI has a very low coefficient of thermal expansion. With 25 × 10−6 K−1 the thermal expansion of the PAI composite containing 20 vol.% graphite and 3 vol.% PTFE is very similar to the one of aluminum (24 × 10−6 K−1 ). The material is designated as “bearing-grade” and is therefore an interesting candidate for the coating of the piston skirts of aluminum pistons (see also Section 18.4.1). Other common automotive applications of all PAI materials are thrust washers, seal rings, sliding vanes, bushings, clutch rollers, gears, valve plates and valve seats. Figure 18.2 shows the specific wear rates of selected bearing grade PAI composites in dry sliding against steel [33]. With one exception, all values lie in the range of 10−7 mm3 /Nm, which is amongst the lowest wear rates known for polymeric composites under the given loading parameters.

18.3.2 Epoxy resins (EP) In 1962, U.S. patent No. 3,293,203 filed by G. F. Paulus of Acheson Industries [35] describes a series of thermosettable resin compositions for forming low friction surface coatings. Among other matrices like a thermosettable silicone resin, the expoxy resin Epi-Rez 201 was filled with low molecular weight PTFE to form a highly wear-resistant coating with good adhesion to steel and a coefficient of friction of only 0.044. Today, epoxy resins are still the most popular matrices for slide coatings and many commercial state-of-the-art products like the Scotchkote Series (3M), DAG 213 (Acheson Industries) or Everlube 9002 (Everlube Products) have an EP matrix. The main reasons for this are the beneficial high mechanical strength and stiffness of epoxy resins, their high chemical resistance, their good processability and their superior performance at temperatures that occur in automotive applications. The most distinct feature of epoxy resins however is the extraordinary wide range of available monomers, which allows for tailoring coatings to meet the demands of individual applications beyond the mere choice of a single (thermoplastic) matrix. EPs consist of two components, one of them being a polyfunctional epoxide that is chemically reacted with a curing agent [36–38]. Today, more than 90% of the world’s overall EP production is based on the diglycidyl ether of bisphenol A (DGEBA). However, composites for high-temperature applications are mainly based on novolac resins since their glass transition temperatures are about 50 ◦ C higher than for DGEBA resins. Novolacs are oligomers of two to five monomers formed by condensation of formaldehyde with phenol and subsequent condensation with epichlorhydrin (Fig. 18.3). Novolacs have superior mechanical and thermal properties as well as

448

Tribology of Polymeric Nanocomposites

Fig. 18.3. Chemical structure of DGEBA (left) and novolac resins (right).

a higher chemical resistance than DGEBA resins [39]. However, their adhesion to metallic substrates does not completely reach the high level of DGEBA resins. Current commercial novolac resins include for example the Dow Epoxy Novolac (D.E.N.) series from The Dow Chemical Company [17]. As curing agents for EPs, organic polyamines or anhydrides of organic acids are available. Since amines generally yield better mechanical properties they are frequently used to cure resins for hightemperature applications. From the chemical point of view, both types of hardeners, i.e. anhydrides as well as amines, can be subdivided into aliphatic and aromatic species. Due to their π-electrons aromatic moieties exhibit higher intrinsic rigidities, stronger non-covalent interactions and higher resistances to solvents. Additionally resins cured with aromatic hardeners show a less pronounced degradation of mechanical properties at elevated temperatures. Overall, resins cured with aromatic amine hardeners exhibit the highest mechanical strength, glass transition temperature and chemical resistance compared to other hardeners. As an example DGEBA cured with diethylene triamine, an aliphatic hardener, a Tg of 112 ◦ C is obtained. Curing the same resin with the aromatic DDS Tg is 150 ◦ C. Furthermore, the size of the monomers has greatly influences the properties of the cured resin. Compact resin molecules as well as compact hardeners generally result in a higher density of covalent bonds. Since their bonding energies are about one to two orders of magnitude higher than those of non-covalent bonds strength and stiffness are higher than for a network formed by molecules with large distances between their epoxy functionalities. Additionally compact species tend to have a higher content of aromatic groups. Following these general guidelines the achievement of high-performance properties by the careful choice of monomers was demonstrated by Zheng et al. [40]. Therefore, the trifunctional DGEBA resin TDE-85 was cured with a series of different amine hardeners (Fig. 18.4). The latter has been designed that the amines are getting more compact from DDM to MPA and that therefore a denser threedimensional network is achieved and the free volume in the cured material is minimized. As can be seen from Table 18.7 this results in an increase of the bulk density which is accompanied by an increase of strength and stiffness while reducing the materials impact strength. The compressive properties, the flexural modulus and the heat distortion temperature, which are essential for the use in highly loaded slide coatings, increase significantly when curing TDE-85 with the smallest hardener (MPA). Compressive strength, compressive modulus and heat distortion temperature are 15–30% higher than for DDS which is commonly considered to be one of the most efficient amine hardeners. When curing CYD-128 with the highly compact dicyane diamide (DICY) hardener, much lower mechanical and thermal properties are obtained. This is attributed to CYD-128 being considerably larger than TDE-85 and having a more flexible molecular backbone, which leads to the formation of a rather wide-meshed network of covalent bonds with bigger, free volumes.

18.4 Nanoparticle-filled Slide Coatings for Automotive Applications This section describes three automotive applications in which coatings with a combined micro- and nano-reinforcement have proven to be superior to conventional micro-particle or fiber-reinforced slide

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Fig. 18.4. Chemical structure formulae of the resins and hardeners used to study structure–property relationships of epoxy resins. Table 18.7. Mechanical and thermal properties of different cured model resins. TDE-85

DGEBA

Epoxy resin hardener

MPD

A-50

DDS

DDM

DICY

Density (g/cm3 ) Tensile strength (MPa) Tensile modulus (GPa) Strain at break (%) Flexural strength (MPa) Compressive strength (MPa) Compressive modulus (GPa) Impact strength (J/m2 ) Heat distortion temperature (◦ C) Glass transition temperature (◦ C) Coefficient of thermal expansion (10−6 /K)

1.369 85.7 5.3 2.5 215 233 7.3 11.9 >250 230 44

1.318 86.6 5.1 2.31 197 228 6.67 10.4 >250 228 47

1.311 75.0 4.3 2.3 133 194 5.59 17.1 214 222 50

1.309 72.2 4.1 1.6 113 203 5.32 14.6 202 214 44

1.215 73.2 2.96 1.36 112 125 – 8.6 – – 62

Source: Data from ref. [40].

coatings: a PAI-based piston skirt coating and tribological coatings for polymer/metal-slide bearings for high-temperature use and for use under boundary or mixed lubrication.

18.4.1 Slide coatings for piston skirts Concerning pistons, at least three separate tribological topics must be discerned. The first one is the piston ring pack (Fig. 18.5) which in most cases consists of three metallic rings [41] : top and second

450

Tribology of Polymeric Nanocomposites Piston skirt with state-of-the-art slide coating based on PAI

Connecting rod journal bearings

Piston ring pack

Fig. 18.5. Tribosystems of an aluminum diesel engine piston where slide coatings can be used to achieve low wear and friction and thus contribute to the improvement of fuel economy. (Courtesy KS Kolbenschmidt GmbH, modified)

compression ring and oil control ring. While the compression rings seal the combustion chamber off the crankcase, is the oil control ring designed to leave an oil film of only a few micrometers thickness on the cylinder wall as the piston descends. All three rings experience severe combined mechanical, tribological and thermal stress and are therefore usually made of hard-coated metals. They can however also be made from high-performance polymers [42]. The second field of interest related to pistons is the pair of connecting rod journal bearings. Here metallic or polymer/metal-slide bearings are currently inserted into the piston during manufacturing. The substitution of these bearings by a slide coating, which could for example be simultaneously applied with the coating of the piston skirt, would reduce the overall number of parts, shorten the manufacturing time and therefore reduce manufacturing costs. The coating would however be subject to compression loadings of up to 2,000 bar and to sliding speeds of up to 20 m/s. The third area of tribological interest is the piston skirt, which lies directly below the ring pack and upon which we will focus throughout the remainder of this section. The piston skirt transmits transverse loads onto the cylinder wall. The secondary motion of the piston, which causes these transverse forces, includes transverse motion and tilting about the main piston axis [43, 44]. This leads to noise generation due to collisions of the piston with the cylinder wall and to severe frictional loss and high wear. The piston skirt/cylinder wall interaction is estimated to contribute 30% to the piston assembly’s frictional loss and is therefore one of the largest single portion of a vehicle’s total frictional loss [6]. Hence, the reduction of piston skirt friction is a very important task in automotive tribology. Although not dedicated foremost to the reduction of friction but rather aimed at the increase of the wear resistance, one of the earliest patents related to piston coatings was issued in 1934 [45]. At that time, light-weight pistons from aluminum alloys were becoming increasingly popular. Although these were light weighted, easily manufactured and had high thermal conductivities, their wear resistance was not as good as for pistons made from heavy metals. Therefore, a layered coating with a hard nickel layer and the subsequent addition of a tin layer which greatly reduced wear and scuffing is described in the patent. In 1957, one of the earliest patents dealing with the reduction of piston skirt/cylinder wall friction by the use of polymers describes the usage of PTFE as a slide coating for the piston skirts [46]. The main requirements for modern piston coatings, wherever they are to be applied, are thermal stability up to 200 ◦ C, high wear resistance, a low coefficient of friction and a lifetime of at least 2,000 h. Since the actual operation temperature strongly depends upon the heat deduction from the piston into the engine block a high thermal conductivity of the coating and a coating thickness of only 5–25 μm are feasible. One of the high-performance polymers that fulfills these requirements is PAI filled with PTFE, MoS2 and graphite [30].

Development of Nanostructured Slide Coatings 2.0

2.0

Optimum graphite content: 7 vol.%

1.5

1.0

0.5

0.5

SCF-content: 9 vol.% 3

5 7 9 11 Graphite content (vol.%)

SCF-content from first step

1.5

1.0

0.0

451

Graphite content: 7 vol.% 0.0

5

9 11 13 7 SCF-content (vol.%)

Fig. 18.6. Reduction of the specific wear rate of a slide coating by a two-stage single-variant optimization of the filler composition.

An important step in the development of a slide coating is the optimization of the individual fillers contents. Usually this is done by the tribological testing of a series of materials in which the content of one filler is varied while the content of all other fillers are kept constant. Figure 18.6 shows the result of such a two-step optimization of the graphite and SCF content of PAI-based slide coating. Although the coating also contains nanoparticles they are considered to be already at their respective optimum contents and have hence not been included in the content variation studies. In the first step, a series of coatings was prepared in which the SCF content was kept constant at 9 vol.% and the graphite content was varied. A series of dry sliding experiments on ring-on-plate tribometer yielded an optimum graphite content of 7 vol.%. In the second step of the optimization procedure, the SCF content was changed while maintaining the graphite content at 7 vol.%. As can be seen from Fig. 18.6 the specific wear rate monotonically decreases with increasing SCF content. Within the scope of this study, the optimum amount for the coating’s micro-fillers therefore are 9 vol.% graphite and 13 vol.% SCF. A very interesting variation of this classical and frequently used single variant method has been proposed by McCook. It utilizes a simplex algorithm in order to incrementally optimize the all filler contents with respect to the specific wear rate [47]. Other methods utilize only one specimen that exhibits a spatial variation of the filler contents [48]. Yet another method, which has become very popular recently, is the predicition of tribological properties and of their dependence on filler content, loading or sliding speed by the use of artificial neural networks (ANN) [49]. The key feature of ANNs is that they predict wear rates or coefficients of friction from an experimental database without the need of explicit functional relationships between the input variables and the wear rate or the coefficient of friction respectively [50, 51].

18.4.2 Polymer/metal-slide bearings Polymer/metal-slide bearings consist of a steel back coated with a tribologically optimized polymer layer. They therefore combine the beneficial mechanical stability, creep resistance and high thermal conductivity of steel with the superior tribological performance of polymeric composites. Today, slide

452

Tribology of Polymeric Nanocomposites 0.4

Coefficient of friction

pv  1 MPa.1 m/s 0.3 Commercial reference

0.2

0.1

0.0

65% frictional loss

PEEK-nano composite

0

25

50

75 100 125 150 175 200 225 250 Temperature (C)

Specific wear rate (106 mm3/N m)

Fig. 18.7. Dependence of the coefficient of friction of both micro- and nano-PEEK at an ambient temperature of 23 ◦ C. 12 pv  1 MPa.1 m/s 9 Commercial reference Wear 75% lifetime 4

6 PEEK-nano composite

3

0.0

0

25

50

75 100 125 150 175 200 225 250 Temperature (C)

Fig. 18.8. Dependence of the specific wear rate of micro- and nano-PEEK at an ambient temperature of 23 ◦ C.

bearings are important components in automobile manufacturing and a medium sized passenger car contains up to 2,000 individual slide bearings. Besides many standard applications, such as door hinges or wipers they are also used in high-temperature, high pv-applications like connecting rods, motor bearings or fuel injection pumps. The following sections deal with three specific automotive applications in which the main requirements are a low coefficient of friction, a low wear rate and high operation temperatures and in which nanoparticle reinforced composites have been found to be superior to conventional micro composites.

18.4.2.1 Use in high-temperature applications In this section we focus on the comparison of the high-temperature tribological performance of the commercial polyetheretherketone (PEEK)-based micro composite Lubricomp-LTW and a nanoparticle-filled composite with similar composition [52, 53]. While the commercial reference contains 10 vol.% SCF, graphite and PTFE, respectively, the nanocomposite contains 10 vol.% SCF, graphite, nano-ZnS and nano-TiO2 , respectively. Both materials were applied to a steel plate with a thickness of 100 μm and subjected to sliding wear against 100Cr6 in the temperature range of 23– 225 ◦ C using the plate-on-ring geometry. Figure 18.7 depicts the coefficient of friction and Fig. 18.8 the specific wear rate of both PEEK composites as a function of the ambient temperature.

Development of Nanostructured Slide Coatings

453

Diesel fuel lubricated journal bearings

Fig. 18.9. Diesel fuel injection pump. (Courtesy Robert Bosch GmbH, modified)

At 23 ◦ C both materials exhibit similar specific wear rates of about 0.6 × 10−6 mm3 /Nm while the nanoparticle-filled composite has a slightly higher coefficient of friction. At higher temperatures, however the nanocomposite exhibits significant advantages over the commercial micro-composite. The coefficient of friction of both materials is initially reduced with rising ambient temperature. At 150 ◦ C which is the Tg of both materials it reaches a minimum of 0.06 for the nanocomposite and 0.10 for Luvocom LTW, respectively. Above 150 ◦ C friction increases continuously with temperature, but in all cases the nanocomposite remains at a significantly lower coefficient of friction. At 225 ◦ C the coefficient of friction of the nanoparticle-filled composite is still only 0.11, which is 65% less than the micro-materials coefficient of friction of 0.32. Starting at an ambient temperature of 100 ◦ C the nanofillers cause a significant reduction of the specific wear rate related to the micro-composite’s wear rate. At a temperature of 225 ◦ C this amounts to a 75% advantage which corresponds to a threefold lifetime prolongation.

18.4.2.2 Use under mixed/boundary lubrication Acceleration resistance contributes up to 35% of a vehicle’s total energy consumption. 80% thereof is caused by the vehicle’s mass. As a rule of thumb a reduction of a vehicle’s weight by 100 kg reduces its fuel consumption by 0.5 l per 100 km. An important mean of reducing vehicle weight is the downsizing of components. This however also forces tribological components to operate under higher pv-products, which can shift the operation conditions into the mixed or boundary lubrication, both of which are usually associated with frictional and wear. In the boundary friction regime, for example, there is no bearing oil film like in the hydrodynamic regime and hence load is carried by asperity contact. In contrast to dry sliding, there is however an adsorbed layer of lubricant molecules on the mating surfaces which protects the asperities from excessive wear. Under boundary lubrication, polymeric slide coatings have many advantages over their metallic counterparts. They have superior anti-seizure and anti-galling properties, a high corrosion resistance and very good dry-running properties. Furthermore, polymeric slide coatings are able to embed foreign particles like oil contaminants or wear debris. One component that operates under very high tribological loading is the fuel injection pump. Due to simplicity of design and manufacturing as well as for reducing costs, there is no separate oil-based lubrication for the journal bearing/camshaft-pairing that is subject to continuous unidirectional sliding (Fig. 18.9). Current pumps rather rely completely on the intrinsic lubrication of the delivered fuel. Since efficient fuel combustion requires high fuel injection speeds, conveying pressures of up to 2,000 bar are used and 3,000 bar are aimed at. Even if the hydrodynamic velocity within the slide bearing is never zero during operation, the formation of a bearing hydrodynamic film can locally be disrupted at such severe loading due to the low viscosity and the poor intrinsic lubricity of the diesel fuel [54]. Hydrocarbons, which constitute the largest fraction of diesel fuel, poorly physisorb to metals and are poor lubricants

Tribology of Polymeric Nanocomposites

100

1.0 Specific wear rate (neat PEEK) Specific wear rate (PEEK-composite) Coefficient of friction (neat PEEK)

10

0.8 0.6

1 0.4 0.1 0.01

0.2 0

5 10 15 20 180 Lubricant supply rate (l V1404/h)

Coefficient of friction

Specific wear rate (106 mm3/N m)

454

0.0 200

Fig. 18.10. Decrease of friction and wear with increasing lubricant rate.

and consequently significant portions of boundary friction may occur in the camshaft bearing. While polymer/metal-bearings counter this by tribologically optimizing the slide coating there are also current attempts to enhance the lubricity of diesel fuels by adding organic acids, organic esters, aliphatic amines [55] or biodiesel [56] to low sulfur petrodiesel. Since PEEK is a common slide coating in the journal bearings of diesel fuel injection pumps its tribological behavior and mixed and boundary lubrication have been studied recently [57]. Additionally, a fully optimized tribological nanocomposite based on PEEK has been studied. This tribological composite contains 10 vol.% SCF graphite nano-ZnS and nano-TiO2, respectively. Using the calibrating fluid V1404 [58] as a lubricant and using the coefficient of friction as indicator for the different lubrication regimes, boundary lubrication and mixed lubrication were selectively engaged in sliding wear tests by applying different, but constant lubricant rates in the low μl/h-range. It is found that friction and wear decreased steadily when applying a higher lubricant rate. Compared to dry lubrication the specific wear rate decreases by one order of magnitude in boundary lubrication and by two orders of magnitude in mixed lubrication (Fig. 18.10). Under boundary lubrication, the wear rate of the PEEK composite is only 25% of the neat matrix. Since this advantage of the composite over the neat matrix vanishes at higher lubricant supply rates it is considered to be due to the solid lubricants of the composite. Since latest results indicate that there is a separate contribution from the classical micro-filler (graphite) and from the nanofillers, the deconvolution of the individual fillers’ contributions is the subject of ongoing investigations. Concerning the wear mechanism of polymeric slide coatings under boundary lubrication there are significant differences to metallic slide bearings. For PEEK [57–59], Nylon 11 [59] and PTFE [60] it has been observed that hydrocarbon lubricants (diesel fuel, paraffin oil, decanoic acid, dodecylamine and cresol) are absorbed into the first 5–10 μm of the matrix. This leads to a plasticization of the polymer matrix that reduces surface hardness and elasticity. Consequently, fatigue failure and crack formation perpendicular to the sliding direction are observed under the shear stress induced by sliding (Fig. 18.11).

18.5 Conclusions Slide coatings combine the high mechanical and thermal stability of metallic sliding surfaces with the superior tribological performance of modern polymeric composites. Therefore, they have become indispensable components in the manufacturing of light-weight, high-power and safe vehicles. With the advent of high-performance composites slide coatings are increasingly used in high-temperature and high-pressure applications, for example, in the engine compartment.

Development of Nanostructured Slide Coatings

455

Fig. 18.11. Formation of fatigue cracks perpendicular to the sliding direction (bottom to top) due to lubricant sorption into the neat PEEK matrix [57].

The future development of polymeric slide coatings is mainly determined by the current necessity to increase fuel economy. Besides avoiding mechanical losses one important way is to reduce the vehicle’s weight. Since modern vehicles already utilize aluminum and polymeric composites to a high extent without increasing the costs beyond a sensible limit, a very prospective way to achieve further weight reduction is the downsizing of individual components. This will require the corresponding tribological systems to operate at higher sliding speeds, pressures and temperatures in order to achieve the required power output. The future development of slide coatings will therefore aim at meeting these demands without reducing the coating’s service lifetime. Concerning polymer/metal-slide bearings the direct application of the slide coating to a metallic bulk instead of joining a separate bearing is desirable. This would e.g. economize the need to join two bearings into each piston (see also Fig. 18.5) and would therefore reduce assembly time and costs. Besides this procedural advantage and the pure enhancement of the mechanical and tribological properties of composites, nanosized fillers can also provide “auxillary” functions. One of them, which is in our opinion not yet fully utilized, is the reduction of a composite’s thermal resistance by the incorporation of a nanofiller with high thermal conductivity (e.g. single-walled carbon nanotubes). In effect the temperature rise in the coating due to the deduction of the frictional heat can be greatly reduced and thermal damage of the polymeric matrix can be prevented.

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