The use of ceramic matrix composites for metal cutting applications

The use of ceramic matrix composites for metal cutting applications

25

Jun Zhao Shandong University, P. R. China

Abstract Al2O3 ceramics retain their hardness at elevated temperatures, are chemically inert to ferrous workpieces and can be used at high cutting speeds. Early ceramic tools lacked toughness and resistance to mechanical and thermal shocks. The need for tougher and more reliable ceramic tools led to better processing techniques and ceramic matrix composites. Ceramic tools are made by hot pressing, hot isostatic pressing or cold isostatic pressing followed by pressureless sintering. There are four families: (1) Al2O3-based ceramics, (2) Si 3N4-based ceramics, (3) sialon-based ceramics and (4) cermet tool materials. They are used for metal cutting after strengthening and toughening, e.g., transformation toughening, particle-dispersion toughening, whisker toughening and the use of nanocomposites. Ceramic inserts are used to machine hard-to-cut materials.

Keywords: Cutting tool materials; Graded ceramic tools; Micro-nanocomposite ceramic tools; Strengthening and toughening mechanisms.

25.1

Introduction

The first modern industrial applications of ceramics as cutting tools occurred in the 1930s (Whitney, 1994). The early pure Al2O3 ceramic tools (with MgO as a sintering aid) were able to retain their hardness at elevated temperatures while being chemically inert to ferrous workpieces (low hardness steels and grey cast irons). These advantages over cemented carbide tools allowed higher cutting speeds. However, the main problem with the early pure Al2O3 ceramic tools was that they lacked toughness and resistance to both mechanical and thermal shocks. Since the 1980s the need to develop tougher and more reliable ceramic tools has brought about developments in both processing techniques and ceramic matrix composites. Hot pressing (HP), hot isostatic pressing (HIP) and cold isostatic pressing (CIP) followed by pressureless sintering (PS) are widely used in the preparation of ceramic tool materials, thanks to advances in sintering furnaces, most of which

Advances in Ceramic Matrix Composites. https://doi.org/10.1016/B978-0-08-102166-8.00025-6 © Woodhead Publishing Limited, 2014.

624

Advances in Ceramic Matrix Composites

are programmable. Currently, there are four distinct families of ceramic tool material: • • • •

Al2O3-based ceramics Si3N4-based ceramics sialon-based ceramics cermet tool materials.

All these types of tool material require various strengthening and toughening methods, e.g., transformation toughening, particle-dispersion toughening, whisker toughening or the use of nanocomposites. The classification and strengthening and toughening of ceramic tool materials are described in Sections 25.2 and 25.3, respectively. Coated tools such as coated carbides are not included in this chapter. Section 25.4 discusses the design and fabrication of graded ceramic tools. Section 25.5 describes the applications of ceramic inserts in machining of hard-to-cut materials such as hardened steels, high-strength steels, high-temperature alloys and nodular cast irons. Future trends are summarized in Section 25.6.

25.2 25.2.1

Classification of ceramic matrix composites (CMCs) for metal cutting applications Overview of metal cutting tool materials

Metal cutting methods such as turning, boring, milling and drilling are industrial processes in which metal is shaped by the removal of unwanted material in the form of a deformed chip. While metal cutting has roots going back to the Industrial Revolution, it continues to develop in response to the everyday needs of a wide range of contemporary industries. The economic importance of metal cutting using machine tools cannot be underestimated. “Today in industrialized countries, the cost of machining amounts to more than 15% of the value of all manufactured products in those countries” (Merchant, 1998). The design of cutting tools has a strong impact on their machining performance. Well-designed tools produce parts with consistent quality and have long predictable useful lives. Cutting tools must be made of materials capable of withstanding the high stresses and temperatures generated during the cutting process. In current machine shop practice, the most common tool materials are high-speed steels (HSSs), powder metallurgy high-speed steels (PM HSSs), cemented carbides (WC), cermets, coated tools (coated carbides and coated cermets), ceramics, polycrystalline cubic boron nitride (PCBN) and polycrystalline diamond (PCD). See Fig. 25.1. Ideally, tool materials should have the following properties (Stephenson and Agapiou, 1997): • •

High penetration hardness at elevated temperatures to resist abrasive wear. Fig. 25.2 shows the hardness of typical tool materials as a function of temperature. High deformation resistance to prevent edges from deforming or collapsing under the stresses produced by chip formation.

The use of ceramic matrix composites for metal cutting applications

Figure 25.1 Historical development of tool materials (Abele, 2007).

Figure 25.2 Hardness of tool materials versus temperature (Almond, 1981).

625

626

Advances in Ceramic Matrix Composites

Figure 25.3 Hardness and toughness of tool materials (Berger et al., 2010).

•

• • • • • •

High fracture toughness to resist the chipping and breakage of edges, especially in interrupted cutting. Fig. 25.3 shows that, for tool materials, hardness and toughness suffer an inverse ratio. A major aim in the development of tool materials is to successfully increase toughness while maintaining hardness. Chemical inertness or low chemical affinity. This relates to a material’s ability to resist diffusion, chemical wear and oxidation wear. High thermal conductivity to reduce cutting temperatures around the tool’s edge. High fatigue resistance, especially for tools used in interrupted cutting. High thermal shock resistance to prevent tool breakage in interrupted cutting. High levels of stiffness to maintain accuracy. Adequate lubricity (low friction) to prevent a built-up edge, especially when cutting soft ductile materials.

As can be seen from Figs. 25.2 and 25.3, no single material exhibits all of the desirable properties. The most suitable tool material for each given cutting process depends upon a number of factors. Fig. 25.4 shows the typical speed ranges considered permissible for given combinations of tool and work materials, an analysis which is based largely on the consideration of temperature resistance and chemical inertness. With the improvements in fracture toughness, resistance to both mechanical and thermal shocks, as well as the advances in computer numerical control (CNC) machine tools and tool clamping systems, the potential applications of ceramic inserts have been extended from continuous turning to interrupted/intermittent turning and milling operations. Furthermore, newly emerged alternatives for ceramic tool materials, such as borides (Fig. 25.5), have considerably broadened the scope of available workpiece materials (Deng and Zhao, 2005).

The use of ceramic matrix composites for metal cutting applications

Figure 25.4 Effect of tool material on cutting speed (Stephenson and Agapiou, 1997). 627

628

Advances in Ceramic Matrix Composites

Figure 25.5 Possible combinations of constituents for ceramic tool materials (Deng and Zhao, 2005).

25.2.2

Al2O3-based ceramic tool materials

The first major development in CMCs for cutting tools was the development of Al2O3/TiC composites (carboxide ceramics) in the early 1970s (Whitney, 1994). For the turning of regular as well as hard ferrous metals, they provided an improved level of finish. Optimization of the composition, for instance through the introduction of new sintering technologies, resulted in further improvements to these cutting tool materials. The dispersion of TiC particles can significantly improve a material’s strength during exposure to high temperatures, with such materials managing temperatures up to 800
C higher than oxide ceramics. Furthermore, fracture resistance and flexural strength can be improved through crack impediment, crack deflection or crack branching. The combination of increased hardness as well as toughness improves a material’s resistance to abrasive and erosive wear considerably. Besides TiC, other hard particles such as TiN, (W,Ti)C, Ti(C,N), TiB2 and SiC are also used to improve the mechanical properties of Al2O3-based ceramic tool materials via particle-dispersion toughening. ZrO2 was often employed as a complementary reinforcing phase to improve the toughness of materials through conversion from the tetragonal (t) phase to the monoclinic (m). The first commercial cutting tools made from a Al2O3/SiC(w) (SiC whisker) composite were introduced in 1985 by the Advanced Composite Materials Corporation (ACMC) and Greenleaf Corporation, with the fracture toughness of these composites found to be double that of nonwhisker-reinforced Al2O3. The mechanical properties of selected Al2O3-based ceramic tool materials are listed in Table 25.1. Usually Al2O3/carbide or Al2O3/nitride composite grades are selected for machining ferrous metals (such as steels and cast irons) at high cutting speeds. This is because of their hardness, wear and heat resistance, chemical stability, as

Table 25.1

Mechanical properties of Al2O3-based ceramic tool materials

Composite

Producer

Grade

Density (g/cm3)

Hardness (HRA)

Al2O3/ZrO2

Comadex

CKZ2

4.0

1800 HV

Al2O3/TiC

SDUa

LT55

4.96

93.7e94.8

900

SDU

FG-1 (graded)

94e95

820

Dijet

CA100

4.2

2130 HV

1000

Sumitomo

NB90S

4.33

95

950

Comadex

CK1

4.2

2100 HV

LP-1

4.08

94e95

800e900

5.2

Fracture toughness MPa$m1/2 4.5

4.0

Al2O3/Ti(C,N)

Sandvik

CC650

4.26

1800 HV

550

4.0

Kennametal

KY1615

4.4

2000 HV

600

6.5

Comadex

CK8

4.6

2300 HV

SDU

AT

4.6

2040 HV

820

7.4

SDU

SG-4

6.65

94.7e95.3

850

4.94

SDU

FG-2 (graded)

94.7e95.5

830

SDU

AWT10

6.65

2350 HV

930

7.55

Al2O3/WC/TiCn

SDU

LWT-1

6.5

2340 HV

840

7.87

Al2O3/SiC(w)

SDU

JX-1

3.63

94e95

800

8.5

Machining grey cast irons, hardened steels and alloy steels

4.8

Sandvik

CC670

3.74

2000 HV

900

8.0

Kennametal

KY4300

3.74

2000 HV

700

7.7

SDU

LP-2

3.94

94e95

700e800

7.8

SDU: Shandong University, China.

Finish and medium cutting of steels and cast irons

5.6

SDU

Al2O3/TiB2/SiC(w)

Applications

5.04

Al2O3/TiB2

Al2O3/(W,Ti)C

a

a

Flexural strength (MPa)

Machining hardened steels

Machining heat-resistant alloys

630

Advances in Ceramic Matrix Composites

well as their ability to achieve a high level of accuracy and finish. Interrupted cuts are only recommended with very small chip sections and strong cutting edge designs, as found in round or square inserts with T-lands. Al2O3/SiC(w) composite grades are used for machining heat-resistant alloys, such as those which are nickel-based, in virtue of their increased resistance to fracturing. By adding micro-sized WC and nano-sized TiC particles into the micro-sized Al2O3 matrix, Al2O3/WC/TiCn micro-nanocomposite ceramic tool material LWT-1 (Table 25.1) was developed in Shandong University. This was achieved through the use of the hot-pressing technique (Zhao et al., 2010a). To prevent the formation of agglomerative TiC nanoparticles (average grain size of 140 nm), the surfactant polyethylene glycol (PEG) and deionized water were used as a dispersant and dispersing medium, respectively. This was done with the aim of obtaining a well reagglomerated and uniform suspension of TiC nanoparticles. These particles were then mixed with micro Al2O3 (average grain size of 0.5 mm), WC powder (average grain size of 0.4 mm) and the sintering additives MgO and NiO. The mixed slurry was ball milled for 48 h and then dried at 330
C in a vacuum. The powder mixture was sieved through a 120 mesh sieve and loaded into a cylindrical graphite die with an inner diameter of 42 mm. The specimens were then sintered via the hot-pressing technique with flowing N2 at a temperature of 1700
C for 10 min under a fixed uniaxial pressure of 30 MPa. Grade AWT10 (Table 25.1) is another micro-nanocomposite ceramic tool material that was hot pressed by adding micro-sized (W,Ti)C and nano-sized Al2O3 particles into a micro-sized Al2O3 matrix (Zhou et al., 2010). For both LWT-1 and AWT10 grades, a mix of intergranular and transgranular fractures were observed on the fracture surface (Fig. 25.6), which indicates the materials’ integrative mechanical properties (Zhao et al., 2010a).

Figure 25.6 Fracture surface of LWT-1. Reprinted from Zhao et al. (2010a) with permission from Elsevier, copyright 2010.

The use of ceramic matrix composites for metal cutting applications

631

25.2.3 Si3N4 and sialon-based ceramic tool materials Si3N4-based ceramics are currently the premier ceramic materials for high-stress, hightemperature applications such as gas turbines, turbochargers, engine valves as well as cutting inserts. This is due to their higher toughness and thermal shock resistance compared to Al2O3-based ceramics. Si3N4-based ceramic tool materials can be classified into: • • •

Si3N4 containing glass-forming sintering aids dispersoid-Si3N4 matrix composites Si3N4-Al2O3 solid solutions (sialons).

The mechanical properties of selected Si3N4-based ceramic tool materials are listed in Table 25.2. Si3N4-based ceramic tools are generally used for machining cast irons, while sialon-based ceramic tools have found appropriate application in the machining of high alloyed grey cast irons and high-temperature alloys such as Inconel 718. Single-phase Si3N4 is a highly covalent compound, which exists in two hexagonal polymorphic crystalline forms, a and the more stable b. The transition from a-Si3N4 to b-Si3N4 is achieved by a solution-precipitation reaction of Si3N4 and molten glass. The strongly covalent bonds of Si3N4 produce a number of desirable engineering properties in this material: high strength, thermal stability up to approximately 1850
C, high oxidation resistance, low thermal expansion coefficient, increased thermal shock resistance and a high Young’s modulus. However, the adverse effect of this bonding is a low self-diffusion coefficient. This makes it difficult to fabricate Si3N4 into a dense

Mechanical properties of Si3N4-based ceramic tool materials

Table 25.2

a

Hardness (HV/ 18.5 kg)

Flexural strength (MPa)

Fracture toughness (MPa$m1/2)

Composite

Producer

Grade

Density (g/cm3)

b-Si3N4

Kennametal

KY1320

3.23

15.5

778

7.6

High speed roughing to finishing of cast irons

b-Si3N4/ TiC

SDUa

MNST28

3.59

17

1090

7.5

Roughing to finishing of cast irons

a0 /b0 sialon

Kennametal

KY1540

3.35

18.24

7.45

General purpose to finish machining of hightemperature alloys

SDU: Shandong University, China.

Applications

632

Advances in Ceramic Matrix Composites

body by solid-state sintering, and as such requires sintering additives (such as Al2O3, Y2O3 and MgO) in order to achieve full density. The addition of dispersed phases, such as transition metal carbides or nitrides (TiC, TiN, HfC, etc.), to a Si3N4 matrix results in increased hardness of the composite. However, for Si3N4-based composites with elongated b-Si3N4 grains, the higher fracture toughness is usually achieved at the expense of a decrease in strength in virtue of the formation of microcracks around the large grains (Chu et al., 1993; Baril et al., 1993). By adding nano-sized Si3N4 and nano-sized TiC particles into the submicrosized Si3N4 matrix, the Si3N4/TiC micro-nanocomposite ceramic MNST28 (Table 25.2) was fabricated by the hot-pressing technique (Zhao et al., 2006). It was found that the rod-like b-Si3N4 grains were characterized by an obvious duplex distribution, with small b-Si3N4 grains embedded in the matrix of large b-Si3N4 grains as shown in Fig. 25.7. This characteristic microstructure is beneficial for the retention of both high-strength and toughness. In the early 1970s, ceramic research showed that aluminum and oxygen could be substituted for silicon and nitrogen, respectively, in the Si3N4 crystal structure. This formed a silicon-aluminum-oxygen-nitrogen solid solution, or what was alternatively termed an expanded lattice b0 -sialon (Whitney, 1994). The general composition of this material is Si6zAlzOzN8z, where z denotes the number of oxygen atoms substituted for nitrogen, which has a limiting value of 4.2 at 1700
C and 2.0 at 1400
C. The composition of a0 -sialon is MxSi12(mþn)Al(mþn)OnN16n, where x  2 M is a metal atom, such as yttrium. It indicates that m (AleN) and n (AleO) bonds replace (m þ n) (SieN) bonds. The grain shapes of a0 -sialon and b0 -sialon are equiaxed and elongated, respectively, hence a0 /b0 -sialons with different properties can be fabricated by adjusting the a0 /b0 ratio to meet the requirements of different cutting conditions.

Figure 25.7 SEM micrograph of eroded surface of a Si3N4/TiC nanocomposite.

The use of ceramic matrix composites for metal cutting applications

633

Figure 25.8 Microstructure of sialon-SiC composite with 10 vol% SiC (SEM, plasma-etched). Courtesy Dr. Bernd Bitterlich.

Adding hard particles to the sialon matrix is another way to further improve a tool’s wear resistance and longevity (Bitterlich et al., 2008). One method of increasing hardness is by adding hard particles, such as SiC, WC, MoSi2 and Ti(C,N), which simultaneously maintains the high fracture resistance of the matrix for the composite material. Fig. 25.8 shows the microstructure of a sialon-SiC-composite with 10 vol% SiC.

25.2.4 Cermet tool materials The term cermet is an acronym derived from the words ceramic and metal, the material’s two primary components. Its other components include carbides, nitrides and carbonitrides of titanium, molybdenum, tungsten, tantalum, niobium, vanadium, aluminum and their solid solutions, with TiN as the major constituent. The metallic binder phase consists of blending nickel alloyed with cobalt, together with the constituents of the ceramic phase, depending on the latter’s solubility. The first “cemented carbides containing TiN” were introduced by Kieffer et al. in 1969 (Whitney, 1994). These materials have a number of advantages over common cemented carbides, including smaller grain size, higher wear resistance and thermodynamic stability. Due to these properties, cermets are regarded as the link between the hard but brittle ceramic cutting tools and the tough but easily worn cemented carbides. Cermets are fabricated with powder metallurgy methods similar to those used for cemented carbides, including liquid-phase sintering. In comparison to cemented carbides, a typical cermet microstructure shows hard, wear-resistant particles imbedded in a ductile binder phase with high toughness and resistance to crack propagation. The TiWCNCo cermet has a core-rim-type hard phase, where the cores are pure cubic TiCN and

634

Advances in Ceramic Matrix Composites

Figure 25.9 SEM image of the microstructure of TiWCN-Co cermet. Courtesy Dr. Daniele Mari.

the rims are TiWCN; its composition changes from the core to the outer grain boundary, as the rims grow on the cores during liquid-phase sintering (Buss and Mari, 2004), as shown in Fig. 25.9. In the past two decades, there have been major developments in cermet tool materials with the introduction of nanocomposite cermets and coated cermets. Zheng et al. (2005) prepared several cermets with nano TiC and TiN additions using spark plasma sintering (SPS). It was found in this study that grain size was reduced by adding nanosized powder. Furthermore, when a cermet was sintered by SPS, the addition of nanopowder could greatly improve the mechanical properties of the cermet. Commercially coated cermets appeared on the market in 1992, expanding their potential for highspeed machining. Various coating techniques, initially physical vapour deposition (PVD) and latterly plasma-assisted chemical vapour deposition (PACVD), have been used for the coating processes, with the coating structure switching from single layer (e.g., TiN) to multi-layer (e.g., TiAlN-TiN, TiN-TiCN-TiN). Table 25.3 lists the mechanical properties of some commercial cermet tool materials. Grades KT315, KT1120 and KT5020 (PVD coated) from Kennametal and grades CT5005, CT5015 and GC1525 (PVD coated) from Sandvik are used to cut low-carbon steels, alloy steels, stainless steels and cast irons.

25.3

Strengthening and toughening of ceramic tool materials

The intrinsic drawbacks of ceramic cutting tools, such as low strength, susceptibility to fracturing and poor thermal shock resistance usually make them prone to damage when machining hard materials, especially under intermittent cutting conditions, leading to a short tool life. During the past three decades much effort has been expended in an

The use of ceramic matrix composites for metal cutting applications

Table 25.3

635

Mechanical properties of cermet tool materials Density (g/cm3)

Hardness (HRA)

Flexural strength (MPa)

Fracture toughness (MPa$m1/2)

Producer

Grade

Mitsubishi

NX2525

92.2

2000

Medium to finish machining of steels, cast irons

NX3035

91.5

2100

Interrupted cutting of steels

Dijet

Applications

LN10

7.2

93.0

1700

7.9

High-speed cutting of steels; finishing of cast irons

CX50

6.7

92.0

1800

8.0

High-speed cutting of general steels

CX75

6.8

92.1

2200

9.0

Medium- and high-speed turning and milling of steels and alloys

CX90

6.9

91.6

2500

10.0

General milling application for steels and alloy steels

attempt to improve the fracture toughness of ceramics. Conventionally, the strengthening and toughening techniques for ceramics, such as particle or whisker dispersion in a matrix, were implemented with the primary aim of redistributing stress at the crack tip. Examples of these strengthening and toughening mechanisms are transformation toughening, microcracking, crack deflection and crack-bridging. However, in recent years, some new strengthening and toughening mechanisms and synergistic mechanisms have been proposed for nanocomposite ceramic tool materials.

25.3.1 Particle-dispersion toughening The toughening of ceramics by particle dispersion was first studied in the 1960s. Selsing (1961) derived an analytical solution for the residual stress around a spherical particle dispersed in an infinite body. According to his theory, if the coefficient of thermal expansion (CTE) of the dispersed secondary particles is larger than that of the matrix (ap > am), the thermal expansion mismatch induced residual micro-stresses. Taya et al. (1990) proposed an analytical model for studying the toughening mechanism for an SiC matrix reinforced by TiB2 particulates, as shown in Fig. 25.10. The fractography evidence for an advancing crack being attracted to adjacent particulates, was

636

Advances in Ceramic Matrix Composites

Figure 25.10 Analytical model of the toughening mechanism by residual thermal stress. q is local average compressive stress, l is semi-infinate crack advances, d is average diameter of particle (Taya et al., 1990).

attributed to the tensile region surrounding a particulate. Countering this effect is the compressive thermal residual stress, which results in the toughening of the composite within the matrix. Taya et al. pointed out that the toughening effect of thermal residual stress is approximately three times higher than that of crack deflection. Zhao and Jin (1996) proposed an analytical model for calculating the thermal residual stress distribution in a particle-dispersed composite. The results of their calculation indicated that the strengthening and toughening effects could be obtained for both cases of ap > am and ap < am. For ap < am, the main toughening mechanisms are crack deflection and the residual stress field, whereas for ap > am, they are crack deflection and microcracking. Xu (2005) investigated the factors affecting the toughening of ceramic composites by thermal residual stress. His work revealed that smaller dispersed secondary particles and a lower particle size ratio (i.e., the ratio of the size of secondary particles to that of the matrix grains) were beneficial for strengthening, whereas larger secondary particles and a higher particle size ratio would result in greater fracture resistance. The trade-off between strength and the fracture toughness should therefore be considered in the microstructural design of ceramic tool materials.

25.3.2

Transformation toughening

The transformation toughening mechanism for ceramic tool materials is generally applied to the ZrO2-containing system. ZrO2 exhibits three well-defined polymorphic phases: monoclinic (m), tetragonal (t) and cubic (c). The room-temperature

The use of ceramic matrix composites for metal cutting applications

637

phase is monoclinic. Upon heating to approximately 1000
C, the monoclinic phase transforms to the tetragonal phase. The transformation to the cubic phase occurs at approximately 2370
C. The transformation toughening of ceramics is based primarily on the t / m transformation, with a martensitic nature. The retention of the t-phase is vital if one is to successfully utilize the transformation toughening phenomenon. For ceramics containing ZrO2, the improvement of fracture resistance has been identified with three principal crack shielding mechanisms: transformation toughening DKcT, transformation-induced microcrack toughening DKcM and crack deflection toughening DKcD (Hannink et al., 2000). Each mechanism’s success depends on the scale, morphology, dispersion and volume fraction of the transforming t-ZrO2. The improvement of toughness resulting from the stress-activated transformation DKcT is commonly expressed as DKcT ¼

hE*eT Vf h1=2 1v

(25.1)

where h is a factor depending on the zone shape at the crack tip, as well as the nature of the stress field in that zone, E* the effective modulus of the material, eT the dilatational strain, Vf the transformed volume fraction of particles, h the width of the transformation zone from the crack surface (i.e., the half-height of the zone), and n Poisson’s ratio. The matrix modulus E* plays an important role in determining the effectiveness of the dilatational strain. Dopants such as MgO, Y2O3 or CeO2 are added to retain the tetragonal phase of ZrO2, hence yttria-tetragonal ZrO2 polycrystal (Y-TZP) and ceria-tetragonal ZrO2 polycrystal (Ce-TZP) were previously used as cutting ceramics in the machining of cast irons. Because of their very low hardness and strength, TZPs cannot be used for high-speed cutting. ZrO2-toughened Al2O3 (ZTA) ceramic tools, usually dispersed with partially stabilized ZrO2 (PSZ) particles, were used for cutting grey cast irons and steels. These have a higher resistance to grooving, notching and chipping wear as a result of their higher fracture toughness and chemical stability compared to Al2O3 tools. During the transformation toughening of ceramics containing ZrO2, the strength and fracture toughness decrease with an increase in temperature. This is because the stability of the tetragonal phase increases, while there is a decrease in the chemical driving force of the t / m transformation. Therefore, the potential application of transformation-toughened ceramic tools is very limited.

25.3.3 Whisker toughening The reinforcement of Al2O3 with single-crystal SiC whiskers was one of the most significant developments for this field during the 1980s. These composites contain up to approximately 45 vol% whiskers, depending on the composition of the matrix. Typically, the whiskers contain b or a mixture of a and b phases of SiC, their dimensions ranging from 0.05 mm to 1.0 mm in diameter and 5 mm to 125 mm in length. Thanks to

638

Advances in Ceramic Matrix Composites

Figure 25.11 Various contributions to steady-state toughness (Evans, 1990).

this successful toughening effect, SiC whisker-reinforced ceramic cutting tools (Table 25.1) have found useful applications in the high-speed machining of nickelbased superalloys. Evans (1990) proposed a unified model for the fracture resistance of whiskerreinforced ceramics that exhibit crack-bridging, as shown in Fig. 25.11. The occurrence of bridging requires microstructural residual stress and/or weak interfaces. The large local residual stresses caused by thermal expansion mismatch and anisotropy are capable of suppressing local crack propagation and therefore may allow ligaments to exist intact behind the crack front. When these ligaments eventually fail in the crack’s wake, energy is dissipated as acoustic waves, which causes toughening. Low fracture energy interfaces (and/or grain boundaries) can cause a crack to deflect along those interfaces, again permitting intact ligaments. As the crack extends, further debonding can occur. Eventually, the bridging material fails, either by debonding or by fracturing. Following this failure, frictional sliding may occur along the debonded surface. The energy dissipated by crack propagation thus includes the energy for debonding the interfaces, the acoustic energy dissipated upon reinforcement failure and the energy dissipated by friction during pull-out. It should be noted that the type of whisker used has a profound effect on the fracture toughness and work of fracture values (Table 25.4), and that small differences in whisker surface chemistry or morphology could be responsible for the effect (Vaughn et al., 1987).

25.3.4

Strengthening and toughening via the use of nanocomposites

Research into ceramic nanocomposites was pioneered by Niihara and Nakahira (1990) (Niihara, 1991), who first revealed that a dispersion of 5 vol% of SiC nanoparticles into Al2O3 increased the room-temperature strength from 350 MPa to 1.0 GPa.

The use of ceramic matrix composites for metal cutting applications

639

Mechanical properties of polycrystalline Al2O3 and Al2O3/SiC(w) composites

Table 25.4

Al2O3 (15008C)

Al2O3 (16508C)

Al2O3 (19008C)

Composite (Silar-SC-1)

Composite (TatehoSCW-1-S)

Young’s modulus (GPa)

371

380

375

375

393

Fracture strength (MPa)

456  40

385  18

253  8

641  34

606  146

Fracture toughness (MPa$m1/2)

3.3  0.2

5.0  0.2

3.7  0.1

4.6  0.2

e

Work of fracture (J/m2)

10

20

39

67

21

Property

Source: Vaugn et al. (1987).

An important indicator of strength increase is the transition in fracture behaviour from intergranular to transgranular crack propagation. The strengthening mechanism is believed to be chiefly due to the generation of thermal dislocations around the second-phase particles. The research by Zhao et al. (2010a) in developing the Al2O3/WC/TiCn micronanocomposite ceramic tool material LWT-1, showed that synergistic strengthening and toughening mechanisms could be introduced by the addition of multiple-sized secondary-phase particles. Fig. 25.12 shows TEM micrographs of LWT-1. It can be seen clearly from the figure that: • • • •

WC and some nano TiC particles were located at the Al2O3 grain boundaries (Fig. 25.12(a)). The Al2O3 matrix grains of the composite were refined by the addition of nano TiC particles. Other smaller nano TiC particles were trapped inside the Al2O3 grains, forming intragranular microstructures as shown in Fig. 25.12(b). The coexistence of both intergranular and intragranular nano TiC particles led to a mixture of intergranular and transgranular fractures (Fig. 25.6).

As shown in Fig. 25.12(b), a regular array of hexagonal dislocations was observed in an Al2O3 grain around an intragranular nano TiC particle. This was a result of the complex residual stress field induced by the thermal expansion mismatch. The dislocations could be locked or pinned by the intragranular nanoparticle and they could also release the tensile residual stresses existing in the matrix and thus improve the strength of the composite. Additionally, intragranular nano TiC particles can also lead to the formation of sub-grain boundaries (Fig. 25.12(c)), which can also

640

Advances in Ceramic Matrix Composites

Figure 25.12 TEM micrographs of Al2O3/WC/TiCn composite LWT-1: (a) intergranular WC and TiC particles, (b) intragranular TiC and dislocations inside an Al2O3 grain, (c) sub-grain boundaries inside an Al2O3 grain and (d) microcrack inside Al2O3 grains. Reprinted from Zhao et al. (2010a) with permission from Elsevier, copyright 2010.

diminish the matrix grains, resulting in the ceramic material’s improved strength, according to the HallePetch relation. The dislocations inside the matrix grains can also increase the flaw tolerance, resulting in a tougher composite. When the tangential tensile stress inside the Al2O3 matrix exceeds the ultimate strength of the Al2O3 matrix, microcracks initiate and subsequently propagate along the radial direction of a secondary-phase particle, further toughening the composite. The direction of a microcrack extension is subject to the interaction of the microcrack front with the complex residual stress field inside the composite, which is always perpendicular to the direction of the maximum tensile stress. When the microcrack front approaches a secondary-phase particle, the propagation direction changes, causing the crack deflection shown in Fig. 25.12(d).

The use of ceramic matrix composites for metal cutting applications

641

Figure 25.13 Crack-bridging. Reprinted from Zhao et al. (2010a) with permission from Elsevier, copyright 2010.

Additionally, crack deflection, crack branching and crack-bridging (Fig. 25.13) have been observed in a Vickers indentation crack extension path. These extrinsic processes can absorb additional amounts of fracture energy, which could significantly improve a composite’s resistance to fractures.

25.4

Design and fabrication of graded ceramic tools

In the last two decades, together with the above micro-heterogeneity ceramic matrix composites, graded ceramic tool materials with macro-heterogeneity have also been developed. The graded composition of materials makes it possible to influence the thermal stress distribution and the local crack resistance in a desired manner, enabling a further improvement to a tool’s thermal shock resistance, and consequently to its lifespan in the cutting process.

25.4.1 Compositional and structural design The concept of functionally gradient materials (FGMs) was transplanted to the design and fabrication of ceramic cutting tool materials by Ai et al. (1998). In contrast to conventional heat-shielding FGMs, whose composition and structure vary unidirectionally, a symmetrical composition distribution was proposed for design-graded ceramic tools. This was so that both surfaces of an insert made by this means could be used as rake faces. A strength-based fracture criterion for evaluating the thermal shock resistance of FGM ceramics was formulated by Zhao et al. (2004). The design rules for FGM ceramics are as follows: both the thermal expansion coefficient a and the thermal

642

Advances in Ceramic Matrix Composites

Figure 25.14 Symmetrical composition distribution.

diffusivity k (¼ l/(cr)) of the centre should be higher than those of the surface, whereas both Young’s modulus E and Poisson’s ratio n of the centre should be lower than those of the surface. Taking the Al2O3/TiC system as an example, the volume fraction of TiC for the two surfaces should be higher than that of the centre and it has the following exponential form (Fig. 25.14): ( ð41  40 ÞðzÞn þ 40 1  z  0 4p ðzÞ ¼ (25.2) ð41  40 Þzn þ 40 0z1 where p represents a reinforcing particle, i.e., TiC. z is a dimensionless coordinate in the thickness direction, n is the distribution exponent, which determines the compositional distribution of the material, f1 and f0 are the volume fractions of TiC of the two surfaces (z ¼ 1.0) and the middle position (z ¼ 0), respectively. In addition, the higher thermal expansion coefficient of the centre compared to the surface may also lead to the formation of residual compressive stresses in the surface region of the FGM compact during fabrication (during cooling from the sintering temperature to room temperature). This process further increases thermal shock resistance.

25.4.2

Fabrication and characterization of graded ceramic tool materials

Five different volume fractions of TiC (30, 40, 50, 60 and 70 vol%) were selected for an Al2O3/TiC FGM ceramic tool material with a nine-layer structure. For this, the volume fractions of TiC in the middle and surface layers were 30 vol% and 70 vol%, respectively. The optimum distribution exponent n ¼ 1.2 was determined with the aim of achieving the highest structural integrity of the compact, i.e., the lowest residual thermal stress (the von Mises stress calculated by the finite element method), in the fabricating process. The composite was hot pressed in flowing nitrogen for 20 min at a temperature of 1700
C under a pressure of 30 MPa. Fig. 25.15 shows the

The use of ceramic matrix composites for metal cutting applications

(a)

643

(b)

Figure 25.15 Al2O3/TiC graded ceramics: (a) cross section, (b) 50 vol% TiC/60 vol% TiC interface (Ai et al., 1998).

nine-layer structure and the 50 vol% TiC/60 vol% TiC interface of the Al2O3/TiC graded ceramics, which has been named FG-1 (Table 25.1) (Ai et al., 1998). By using the same method, the Al2O3/(W,Ti)C graded ceramic tool material FG-2 was also developed (Zhao and Ai, 2006b).

25.5

Application of ceramic inserts in the machining of hard-to-cut materials

25.5.1 The essentials of tooling technology for metal cutting Indexable inserts with various shapes and sizes are widely used in metal cutting processes. The ISO standard provides an identification system to describe the features of inserts. The identification consists of up to ten symbol codes. Each code defines a feature of the insert, such as its shape, clearance angle, tolerance class, style, size, thickness, cutting point, etc. The insert shape has an influence on insert strength. As shown in Fig. 25.16, the greater the included angle at the insert tip, the greater the strength. The round insert and the 100
corner of the first diamondshaped insert are the strongest. Because of the higher cutting forces and the possibility of chatter, these inserts are more limited in use than the square shape. Therefore, for general use, the square insert is the most practical. Triangle and diamond inserts should only be used when a square insert cannot, such as when machining a corner or a shoulder.

644

Advances in Ceramic Matrix Composites

Figure 25.16 Relation between insert shape and strength.

For indexable ceramic inserts with rounded corners, the dimensions of inserts without a fixing hole and with a cylindrical fixing hole are specified in the standards ISO 9361-1:1991 and ISO 9361-2:1991, respectively. The most common shapes for ceramic inserts are squares (S), triangles (T), diamonds (C for 80
and E for 75
) and round (R) as shown in Fig. 25.17. Fig. 25.18 illustrates the methods of mounting

(a)

(b)

Figure 25.17 Cutting inserts: (a) ceramic inserts (Shandong University), (b) cermet inserts (Kennametal).

The use of ceramic matrix composites for metal cutting applications

645

Figure 25.18 Methods of mounting inserts on toolholders for turning operations: (a) clamping for inserts without a fixing hole, (b) wing lockpins for inserts with a fixing hole.

Figure 25.19 Milling cutter with square inserts (Kennametal).

inserts on toolholders for turning operations. A milling cutter with square inserts is shown in Fig. 25.19. Flank and crater wear, as shown in Fig. 25.20, are the most important indicators of tool abrasion, with the former being the most commonly monitored. The criteria recommended by ISO3685:1993 to define the effective tool life of ceramics are: • • •

catastrophic failure VBB ¼ 0.3 mm, if the flank is evenly worn in region B VBB,max ¼ 0.6 mm, if the flank is unevenly worn in region B.

To promote longer tool life, the cutting edge of the ceramic insert must be reinforced by appropriate edge preparation, as shown in Table 25.5. This can range from a small hone for finishing, to a T-land measuring 0.1 mm wide by 20
for semi-finishing. Combinations of lands and hones may also be used.

646

Advances in Ceramic Matrix Composites

Figure 25.20 Types of tool wear according to standard ISO3685:1993.

25.5.2

Machining of hardened steels

Traditionally, the machining of hardened steel components (45e65 HRC) has been the domain of grinding operations. The turning of hard steels has attracted significant attention in the last 30 years, a method which provides not only much higher efficiency but also greater flexibility (it is possible to carry out internal, external and face turning in just one fixation of the workpiece, something which is rarely achievable in grinding) as well as the possibility of environmentally benign dry machining. PCBN cutting tools are usually used in hard machining because of their abrasion resistance combined with high-temperature stability, albeit at a fairly high cost. Al 2O3-based ceramics such as Al2O3/TiC and Al2O3/(W,Ti)C ceramics are possible alternatives to PCBN tools in hard turning. Cutting performance data comparing the Al2O3/(W,Ti)C ceramic tool AWT10 with a competitor’s ceramic tool in the turning of surface hardened AISI1045 steel (50e52 HRC) under a feed rate f ¼ 0.1 mm/rev and depth-of-cut ap ¼ 0.1 mm are listed in Table 25.6. A photo taken during the cutting process is shown in Fig. 25.21, where one can see the melting chip spraying like a flame. The excellent cutting performance of AWT10 grade ceramic tools is attributed to its high integrative thermo-mechanical properties derived from the synergistic strengthening and toughening mechanisms through the addition of micro-sized

Edge preparation parameters for ceramic inserts

Approach

Parameters

Application

Hone

rn ¼ 0.02e0.05 mm

For light finishing and grooving

T-land

bg1  gn1 ¼ 0.1 mm  20


General purpose for turning and light milling

T-land and hone

1. General roughing bg1  g01 ¼ 0.2 mm  20
rn ¼ 0.05e0.1 mm 2. High-speed roughing bg1  g01 ¼ 0.3e0.5 mm  30
rn ¼ 0.05e0.1 mm

Used where more protection is needed than T-land, such as in scale and light interruptions, hard turning

Double T-land and hone

1. Heavy rough turning bg1  g01 ¼ 0.7 mm  20
b0g1  g01 ¼ 0.2 mm  45
rn ¼ 0.05e0.1 mm 2. Rough milling bg1  g01 ¼ 0.5 mm  30
b0g1  g001 ¼ 0.2 mm  60
rn ¼ 0.05e0.1 mm

Generally used on larger inserts in heavy roughing operations

The use of ceramic matrix composites for metal cutting applications

Table 25.5

647

648

Advances in Ceramic Matrix Composites

Table 25.6 Comparison of cutting performance in turning hardened AISI1045 steel

Tool life

Surface roughness

Competitor

AWT10

v ¼ 190 m/min

T ¼ 11 min

T ¼ 18 min

v ¼ 240 m/min

T ¼ 7 min with edge chipping and flaking

T ¼ 12 min

v ¼ 190 m/min

Ra ¼ 0.8 mm

Ra ¼ 0.4 mm

v ¼ 240 m/min

Ra ¼ 0.8 mm

Ra ¼ 0.4 mm

Figure 25.21 High-speed turning of hardened steel with Al 2O3/(W,Ti)C insert grade AWT10.

(W,Ti)C and nano-sized Al2O3 particles into the micro-sized Al2O3 matrix. In addition, to continuous hard turning, Al2O3-based micro-nanocomposite ceramic tools AWT10, LWT-1 and the graded ceramic tool FG-2, all developed in Shandong University, were successfully used in the intermittent turning of hardened steels. These tools exhibited a longer tool life than common ceramic tools (Zhao and Ai, 2006b; Zhao et al., 2010b; Zhou et al., 2010).

25.5.3

Machining of high-strength steels

High-strength low-alloy steels such as 300 M and AISI4340 have a very good combination of toughness, fatigue resistance and ductility, and are therefore appropriate for use in applications that require particularly high levels of strength, such as aircraft landing gear, high-strength bolts, power transmission gears, shafts and airframe parts. There have been a large number of studies into the turning of high-strength steels with Al2O3-based ceramic tools. These have focused primarily on the tools’ wear resistance and the integrity of the machined surface (Lima et al., 2005; Grzesik, 2009). Fig. 25.22

The use of ceramic matrix composites for metal cutting applications

649

(a)

(b)

Figure 25.22 Wear patterns of Al2O3/TiC ceramic tool in turning 300 M steel at: cutting speed, v ¼ 590 m/min; feedrate, f ¼ 0.1 mm/rev; and depth of cut, ap ¼ 0.15 mm. (a) Rake face, (b) flank face.

shows the wear patterns of Al2O3/TiC ceramic tools (developed in Shandong University) in the high-speed turning of 300 M steel, with the wear mechanisms identified in the machining tests. These include abrasion, adhesion, built-up edge, transferred layers and tribochemical effects.

25.5.4 Machining of high-temperature alloys Nickel-based superalloys (i.e., Inconel 718, Incoloy 901 and Waspaloy) have been widely employed in the aerospace industry, in particular for the sections of gas turbine

650

Advances in Ceramic Matrix Composites

engines exposed to great heat. However, they are classified as difficult-to-cut materials due to their high strength, tendency to harden during machining and low thermal conductivity, all of which lead to high temperatures at the tool/workpiece interface and a short tool life. SiC-whisker-reinforced Al2O3 ceramic (e.g., grades CC670 and JX-1 in Table 25.1 and WG-300 made by Greenleaf Corporation) and sialon-based ceramic (e.g., grade KY1540 in Table 25.2 and CC6060 made by Sandvik Corporation) inserts have proven to be the best choices for machining nickel-based superalloys (Zhao et al., 1997). As shown in Fig. 25.23, the main failure modes of Al2O3-SiC(w) tools are depth-ofcut notching, crater wear and flank wear, with the former being the most predominant. When applied incorrectly, edge chipping, flaking and even fracturing may occur. Adhesive, diffusion and abrasive wear mechanisms were identified in the turning of Inconel 718 alloy with grade KY1540 (Zheng et al., 2010). Besides turning, the new sialon grade CC6060 from Sandvik has been applied in turn-milling operations

Figure 25.23 Tool wear micrographs of grade KY1540 in turning alloy Inconel 718 at v ¼ 300 m/min, f ¼ 0.3 mm/rev and ap ¼ 0.1 mm: (a) wear patterns, (b) high resolution image of rake face (Zheng et al., 2010).

The use of ceramic matrix composites for metal cutting applications

651

Table 25.7 Comparison of cutting performance in turn-milling a nickel-based alloy CC6060

Carbide

Application

Conventional milling

Climb milling

Coolant

Dry

Wet

Insert

RNGN 120700E 6060

CM300-1204E-MM 2040

Cutting speed, v (m/min)

1000

30

Diameter

63

63

Feed rate, fz (mm/tooth)

0.1

0.3

Number of teeth

4

6

Depth of cut, ap (mm)

1.5

2

35

35

Metal removal rate, Q (cm /min)

106

19

Tool life (min)

4

25

424

477

Radial immersion, ae (mm) 3

3

Total material removed, Qt (cm )

for nickel-based alloy gas turbine casings. This tool has achieved much higher cutting speeds than carbide tools, as well as far greater machining efficiency (Table 25.7). It also shows excellent resistance to notch wear and allows greater depth of cut compared to other ceramic grades. CMC cutting tools have also been used in the machining of other materials, such as chromium-based alloys, carburized steels used in welding diesel engine exhaust valves, net-formed gears (Zhao and Ai, 2006a), cast iron, etc. However, discussion of these applications has been omitted due to the text length limitation.

25.6

Future trends

As a result of advances in cutting tool and machine tool technologies, high-speed machining (HSM) has become an established technology for machining a wide variety of metallic and non-metallic workpieces. Many advantages of HSM have been cited, such as high metal removal rates, good surface quality, low cutting forces, high exciting frequencies and heat dissipation through chips, among others (Schulz, 1999). Notwithstanding the advances in optimizing compositions, fabrication (including insert edge preparation) and strengthening and toughening techniques for CMC cutting tool materials, their applications in HSM remain limited due to their relatively low fracture toughness and thermal shock resistance in comparison with carbide tools.

652

Advances in Ceramic Matrix Composites

In response to the HSM requirements of difficult-to-cut materials, the following issues should be addressed in relation to CMC cutting tools: • • • • • •

The composition should be based on a thermodynamic evaluation of the chemical compatibility of the tool/workpiece combination. There should be a micromechanical investigation of the relation between microstructure and properties. This is expected to provide a feasible microstructure design for the tool materials via property subscription (or customization). There should be a quantitative description of the synergistic strengthening and toughening effects introduced by the addition of secondary-phase particles with multiple sizes. The composition and grain size of graded nanocomposite ceramics should be altered with the aim of resisting the thermochemical and mechanical loads that occur during cutting, i.e., there should be self-adaptability. Particle dispersion, mixing techniques as well as the optimization of sintering processes and parameters should be used. Effective ceramic coating technologies should be used to increase the wear resistance of cutting tools.

Acknowledgments Financial support provided by the National Basic Research Program of China (No. 2009CB724402) and the National Natural Science Foundation of China (No. 50875156 and 51175310) are acknowledged. The author is grateful to Prof Dr-Ing Eberhard Abele, from PTW, TU Darmstadt, Darmstadt, Germany, Dr Bernd Bitterlich from CeramTec GmbH, Plochingen, Germany, and Dr Daniele Mari from Ecole Polytechnique Fédérale de Lausanne, Switzerland for permitting use of their images.

References Abele, E., 2007. Werkzeugmaschinen und Industrieroboter, AuszugausdemVorlesungsskript. Institut f€ur Produktionsmanagement, Technologie und Werkzeugmaschinen, TU Darmstadt, Germany, p. 45. Ai, X., Zhao, J., Huang, C.Z., Zhang, J.H., 1998. Development of an advanced ceramic tool material-functionally gradient cutting ceramics. Mater. Sci. Eng. A 248, 125e131. Almond, E.A., 1981. Towards improved tests based on fundamental properties. In: Proceedings of the International Conference on Towards Improved Performance of Tool Materials. National Physical Laboratory and Metals Society, Teddington, Middlesex, UK, pp. 161e169. Baril, D., Tremblay, S.P., Fiset, M., 1993. Silicon carbide platelet-reinforced silicon nitride composites. J. Mater. Sci. 28, 5486e5494. Berger, C., Scheerer, H., Ellermeier, J., 2010. Modern materials for forming and cutting tools e overview. Materialwiss Werkstofftech 41, 5e17. Bitterlich, B., Bitsch, S., Friederich, K., 2008. SiAlON based ceramic cutting tools. J. Eur. Ceram. Soc. 28, 989e994.

The use of ceramic matrix composites for metal cutting applications

653

Buss, K., Mari, D., 2004. High temperature deformation mechanisms in cemented carbides and cermets studied by mechanical spectroscopy. Mater. Sci. Eng. A 370, 163e167. Chu, C.Y., Singh, J.P., Routbort, J.L., 1993. High-temperature failure mechanisms of hot-pressed Si3N4 and Si3N4/Si3N4-whisker-reinforced composites. J. Am. Ceram. Soc. 76, 1349e1353. Deng, J.X., Zhao, J., 2005. Handbook of CNC Tool Materials. Machine Press, Beijing, China. Evans, A.G., 1990. Perspective on the development of high-toughness ceramics. J. Am. Ceram. Soc. 73, 187e205. Grzesik, W., 2009. Wear development on wiper Al2O3-TiC mixed ceramic tools in hard machining of high strength steel. Wear 266, 1021e1028. Hannink, R.H.J., Kelly, P.M., Muddle, B.C., 2000. Transformation toughening in zirconiacontaining ceramics. J. Am. Ceram. Soc. 83, 461e487.  Lima, J.G., Avila, R.F., Abr~ao, A.M., Faustino, M., Davim, J.P., 2005. Hard turning: AISI 4340 high strength low alloy steel and AISI D2 cold work tool steel. J. Mater. Proc. Technol. 169, 388e395. Merchant, M.E., 1998. An interpretive look at 20th century research on modeling of machining. Mach. Sci. Technol. 2, 157e163. Niihara, K., 1991. New design concept of structural ceramics e ceramic nanocomposite. J. Ceram. Soc. Jpn. 99, 974e982. Niihara, K., Nakahira, A., 1990. Particle strengthened oxide ceramics e nanocomposites. In: Vincenzini, P. (Ed.), Advanced Structural Inorganic Composites. Elsevier, Trieste, pp. 637e664. Schulz, H., 1999. The history of high-speed machining. Revista de Ciência Tecnologia 13, 9e18. Selsing, J., 1961. Internal stresses in ceramics. J. Am. Ceram. Soc. 44, 419e421. Stephenson, D.A., Agapiou, J.S., 1997. Metal Cutting Theory and Practice e Manufacturing Engineering and Materials Processing. Marcel Dekker. Taya, M., Hayashi, S., Kobayashi, A.S., Yoon, H.S., 1990. Toughening of a particulate reinforced ceramic matrix composite by thermal residual stress. J. Am. Ceram. Soc. 73, 1382e1391. Vaughn, W.L., Homeny, J., Ferber, M.K., 1987. Processing and mechanical properties of SiC whisker/alumina matrix composites. Ceram. Eng. Sci. Proc. 8, 848e859. Whitney, E.D., 1994. Ceramic Cutting Tools. Noyes Publications, Park Ridge, New Jersey. Xu, C.H., 2005. Effects of particle size and matrix grain size and volume fraction of particles on the toughening of ceramic composite by thermal residual stress. Ceram. Inter. 31, 537e542. Zhao, H., Jin, Z.Z., 1996. Analysis of residual stress and toughening mechanism for particulate composites. J. Chin. Ceram. Soc. 24, 491e497. þ514 (in Chinese). Zhao, J., Ai, X., 2006a. Failure modes and mechanisms of functionally gradient ceramic tools with high thermal shock resistance. In: Proceedings of MSEC2006, Ypsilanti, Michigan, pp. 1e8. Zhao, J., Ai, X., 2006b. Fabrication and cutting performance of an Al2O3-(W, Ti)C functionally gradient ceramic tool. Int. J. Mach. Mach. Mater. 1, 277e286. Zhao, J., Deng, J.X., Zhang, J.H., Ai, X., 1997. Failure mechanisms of a whisker-reinforced ceramic tool when machining nickel-based alloys. Wear 208, 220e225. Zhao, J., Ai, X., Deng, J.X., Wang, Z.X., 2004. A model of the thermal shock resistance parameter for functionally gradient ceramics. Mater. Sci. Eng. A 382, 23e29.

654

Advances in Ceramic Matrix Composites

Zhao, J., Ai, X., L€u, Z.J., 2006. Preparation and characterization of Si3N4/TiC nanocomposite ceramics. Mater. Lett. 60, 2810e2813. Zhao, J., Yuan, X.L., Zhou, Y.H., 2010a. Processing and characterization of an Al2O3/WC/TiC micro- nano-composite ceramic tool material. Mater. Sci. Eng. A 527, 1844e1849. Zhao, J., Yuan, X.L., Zhou, Y.H., 2010b. Cutting performance and failure mechanisms of an Al2O3/WC/TiC micro- nano-composite ceramic tool. Int. J. Refract. Met. Hard. Mater. 28, 330e337. Zheng, G.M., Zhao, J., Song, X.Y., et al., 2010. Ultra high speed turning of Inconel 718 with sialon ceramic tools. Adv. Mater. Res. 126e128, 653e657. Zheng, Y., Wang, S.Y., You, M., Tan, H.Y., Xiong, W.H., 2005. Fabrication of nanocomposite Ti(C,N)-based cermet by spark plasma sintering. Mater. Chem. Phys. 92, 64e70. Zhou, Y.H., Zhao, J., Ai, X., 2010. Cutting performance and wear mechanisms of an Al2O3-based micro-nano-composite ceramic tool. Key Eng. Mater. 443, 244e249.