Influence of Fine Zirconia Particle Shot Peening on Sliding Wear of Zirconia-Silicon Carbide Composites

In this paper, the sliding contact fatigue wear performance of shot-peened zirconia-silicon carbide composite (ZrO2/SiC) plates in contact with silicon nitride balls under compressive residual stress in dry conditions was investigated in order to improve the wear resistance of ZrO2/SiC friction parts. The wear resistance of ZrO2/SiC plates after shot peening was higher than that of plates not treated with shot peening in sliding wear testing under Hertziancontact. Due to fine Zirconia particle shot peening, the tetragonal phase crystal structure in ZrO2 in the near-surface of ZrO2/SiC plates was changed, and 1100 MPa compressive residual stress could be introduced into the near-surface layer of ZrO2/SiC plates. The compressive residual stress was determined to be the main factor in the improvement of the sliding wear resistance of ZrO2/SiC plates.


Introduction
Zirconia (ZrO 2 ) composites have great potential as moving parts in special situations for machine elements or medical apparatus.They have low densities, high hardness, high temperature durability and biocompatibility [1] [2] [3].Particularly the wear resistance is one of the most important properties for moving parts such as bearing or joints.Because severe wear at the contact areas in friction zones of moving parts affects the device's lifespan and stable movement.nuous micro-fracturing from many cracks, in fact, sliding wear related to fracture toughness.Hokkirigawa [7] proposed that the sliding wear of ceramics is related to both K eff and P max and crack length.
In order to improve the friction surface of ZrO 2 composites reinforced by silicon carbide (ZrO 2 /SiC) for practical use, this study focused on shot peening (SP).SP is a well-known surface treatment technique for improving fatigue strength of metal parts.In a typical SP process, a stream of small, hard spheres is shot at a treated surface.After SP, compressive residual stress is generated underneath the treated surface, due to localized plastic deformation in the nearsurface layer.Pfeiffer et al. [8] found that compressive residual stress could also be introduced into the near-surface of silicon nitride (Si 3 N 4 ) using a novel SP method.As a result of SP effects, the resistance of the bearing raceway against surface fatigue damage (severe pitting and chipping) increased [8].Takahashi et al. [9] [10] [11] [12], in addition, reported that the compressive residual stress occurred in the near-surface region of shot-peened partially-stabilized zirconia (PSZ) [9], Si 3 N 4 [10] [11], or Al 2 O 3 [12].The compressive residual stresses at the PSZ and Si 3 N 4 surfaces after SP were approximately 1400 MPa and 880 MPa, respectively.The compressive residual stresses significantly increased PSZ's fracture toughness and bending strength [9].Koike et al. [13] stated the wear durability of PSZ after SP was better than that of PSZ without SP.However, the mechanism behind this increase in wear durability of ZrO 2 /SiC composites under compressive residual stress by SP stress was not clear.Some researchers have reported phase transformation or domain switching as a result of the application of tensile or compressive stress.Tetragonal-to-monoclinic phase transformation [14] [15] and ferroelastic domain switching [16] [17] are well-known mechanisms for toughening of ZrO 2 .Mc Meeking et al. [15] presented that tetragonal-to-monoclinic phase transformation, for one, prevents crack propagation at the crack tip because of the crack closure effect.Kiguchi et al. [16] and Virkar et al. [17] reported the second mechanism as reorientation of ferroelastic domains by externally applied stress.Kiguchi et al. [16] stated that the application of compressive stress converted the c axis into an a axis in lattice constants in the tetragonal phase of ZrO 2 .
As mentioned, the wear properties of shot-peened PSZ were investigated previously [13].However, the effects of SP on the wear resistance of ZrO 2 /SiC composites have not been investigated yet.In addition, microstructural changes after SP, such as domain switching, are unclear.In this work, therefore, the sliding wear properties of the shot-peened ZrO 2 reinforced by silicon carbide (ZrO 2 / SiC) were examined under dry conditions.The near-surface of the ZrO 2 / SiC plates was examined by X-ray measurements in order to explore their microstructural changes after SP.

Materials and Shot Peening Procedure
ZrO 2 reinforced by silicon carbide (ZrO 2 /SiC) was selected as the test material.
Wear test specimens were fabricated from ZrO 2 powder containing 3 mol%Y 2 O 3 and 20 vol% SiC powders.After mixing of ZrO 2 /SiC with ethanol by ball milling for 24 h, the powder was dried in a vacuum chamber.The dry powder was hotpressed under vacuum at 1450˚C and 30 MPa for 1 h.The hot-pressed materials were cut into plates.The size of the ZrO 2 /SiC plates was 25 mm × 25 mm × 4 mm (length × width × thickness).The density of this material was 6.05 g/cm 3 .ZrO 2 /SiC plate specimens with and without SP are referred to as SP and Non-SP specimens.For SP, ZrO 2 beads with a diameter of 180 μm, air pressure of 0.2 MPa, and peening time of 20 s were used.SP coverage was approximately 200%, meaning that the complete ZrO 2 /SiC plate surface was shot-peened twice.The brittle ZrO 2 /SiC were not cracked by the shots when the air pressure was lower than 0.2 MPa.It was suitable condition for compressive residual stress by SP.Finally, after SP, ZrO 2 /SiC plate surfaces were polished with a 0.1 μm diameter diamond solution to truncate the edges on dimples caused by SP.The surface roughness of all samples was measured using a profilometer, with three repeated measurements per sample.The Vickers hardness (HV) of Non-SP and SP plates was measured by a hardness tester using a load of 98 N and indentation time of 20 s.

Sliding Contact Wear Test Setup
Figure 1 illustrates the ball-on-plate sliding wear test at room temperature.Si 3 N 4 balls (grade 3) with 4.76 mm diameter and Vickers hardness of 1600 HV were used as wear counterparts.Sliding wear tests were performed by using a friction wear test machine in reciprocation mode, with a reciprocation length of 10 mm.
The vertical loads Q ranged from 2.94 to 9.80 N, corresponding to a mean Hertzian contact pressure (P mean ) ranging between 816 and 1219 MPa.The maximum Hertzian contact pressures P max ranged between 1220 and 1830 MPa.Values for P mean and P max were calculated from the following equations [18].
mean max mean 2 , 1.5 π In the above, ν 1 and ν 2 are the Poisson's ratios for ball and plate materials, respectively; ν 1 = 0.28 and ν 2 = 0.28.E 1 and E 2 are the Young's moduli for ball and plate materials, respectively; E 1 = 300 GPa and E 2 = 214 GPa.Q [N] is the vertical load, a [m] is the contact area radius, and R is the ball radius.The sliding velocity and frequency were 0.033 m/s and 1.7 Hz, which corresponds to 100 reciprocation cycles per minute.The total wear path length and total duration were 800 m and 400 min, respectively.In order to calculate the wear volume (W vol ) of ZrO 2 /SiC plates after the test, the wear depth (W dep ) and width (W wid ) were measured at three different areas of each specimen using a profilometer.Equation (3) was used to calculate W vol .In the above, the cross-sectional area of the wear groove was approximated as a triangular area (=0.5 × W dep × W wid ), which was multiplied by the reciprocation length (10 mm).The ball and plate specimens prior to and after the tests were observed using an optical microscope with polarized light.

XRD Measurements
To evaluate residual stress and crystal structures at the near-surface of ZrO 2 /SiC plates, XRD measurements were taken.Table 1 lists the conditions for measurement of residual stress, which was estimated using the 2θ-sin 2 Ψ method.The CrKα wavelength was 2.29Å.Tanaka et al. [19] stated the penetration depth of CrKα radiation was estimated as 2 -3 μm.

Surface Observation after Shot-Peening
Figure 2 shows optical microscope and scanning probe microscope (SPM) images of Non-SP and SP plates.Submicron-sized dimples were formed on the surface of the SP ZrO 2 /SiC plates by the impact of the ZrO 2 beads.Table 2 shows the average roughness (R a ) and maximum roughness height (R z ) of Non-SP and SP plates.The R a values of the SP plates after polishing and the Non-SP plates were identical.The R z of the SP plate after polishing was0.73 μm, slightly higher than that of the Non-SP plate (R z = 0.57 μm), due to the fact that the submicron-sized dimples induced by SP were not completely removed by polishing.

Hardness
The HV of Non-SP and SP ZrO 2 /SiC plates was 1260 HV and 1398 HV, respectively, similar to the HV of Non-SP PSZ and SP ZrO 2 plates (1293 HV and 1328 HV, respectively) [13].SP thus caused an increase in HV of both materials [9] [13].It was thought that the hardness of both materials increased because strain hardening or recrystallization occurred due to SP. Figure 3 shows microscopic images of Vickers indentations on the surfaces of Non-SP and SP ZrO 2 /SiC plates.The apparent fracture toughness of the Non-SPZrO 2 /SiC plate was calculated 8.9 MPa•m 0.5 by Indentation Fracture method.However, no radial cracks were formed on the SP ZrO 2 /SiC plates due to the effects of compressive residual stress, indicating that the apparent fracture toughness of ZrO 2 /SiC plates was improved by SP.

XRD Measurements
The measured value of compressive residual stress on the surface of SP ZrO  compressive residual stress on the surface of PSZ was 1400 MPa [9].Thus, the compressive residual stress of ZrO 2 /SiC was lower than that of PSZ, even though the SP conditions were almost the same.It is thought that the SiC particles between the grain boundaries of ZrO 2 locally restrained plastic deformation of ZrO 2 .
Figure 9 show the XRD profiles in the near-surface regions of ZrO 2 /SiC plates.The monoclinic (-111) peak after SP was only slightly detected.This means that tetragonal-to-monoclinic phase transformation was not main reason of large compressive stress on the ZrO 2 /SiC plate surfaces in the SP condition.
However, The peaks around 2θ = 35˚, corresponding to tetragonal phases in ZrO 2 crystal structure, changed after SP [20].The relative integrated intensities of the tetragonal (002) peak and the tetragonal (200) peak changed after SP, as shown in Figure 9.The peak intensity ratios (X) for Non-SP and SP specimens were calculated according to Equation (4).was 0.303 and 1.412, respectively.The X for the SP plates was therefore 466% higher than that for Non-SP plates.Thus, the lattice constants of tetragonal phase in ZrO 2 were changed due to SP.This means that the lattice strain in the tetragonal phase in ZrO 2 increased after SP, which is one of the reasons that large compressive residual stress could be introduced into the near-surface of the ZrO 2 specimens.This phenomenon was considered to be the switching of lattice constants (a and c in Figure 10) due to compressive contact stress during SP, likely causing toughening in ZrO 2 due to ferroelastic domain switching [16].
Upon compressive residual stress generation in ZrO 2 , several mechanisms are proposed to take place; plastic deformation, phase transformation or domain switching.In ZrO 2 /SiC plates under SP conditions, it is thought that domain switching was one of the main mechanism of residual stress generation.Virkar et al. stated that the application of compressive stress exceeding 1650 MPa along the c axis converts the c axis to an a axis, while one of the a axes converts to a c axis [17].Tanaka et al. [19] and Scott [21] reported that the lattice constants in tetragonal crystalline PSZ were a = 0.510 nm and c = 0.519 nm, respectively.In fact, due to contact stress of SP, the c axis in the tetragonal phase converts to an a axis, and correspondingly the a axis stretched into a c axis.As the c axis of the tetragonal phase in ZrO 2 was shortened by 0.009 nm along the compressive direction, the lattice constant changed: the (200) c axis was converted to (002) as shown in Figure 9. Thus ferroelastic domain switching as the lattice constant change in the tetragonal phase in ZrO 2 affects the large compressive residual stress generation.

Conclusions
In order to improve the sliding fatigue wear resistance of Zirconia-Silicon Carbide Composites (ZrO 2 /SiC) for frictional parts, the surface of ZrO 2 /SiC plates was strengthened by shot peening (SP).Shot-peened plates were evaluated through sliding wear tests using a mean Hertzian contact pressure ranging from 816 to 1219 MPa, and the compressive residual stress and profile were examined using X-ray diffraction.From the obtained experimental results, the following conclusions were drawn:

Figure 1 .
Figure 1.Schematic illustration of the sliding wear test in reciprocation mode, and shot peening pattern.

Figure 4 (
Figure 4(a) shows optical microscope image of friction surface on the raceway of shot-peened ZrO 2 /SiC plate after the wear test at P mean = 967 MPa.Microplowings can be observed on both the SP and Non-SP plates.These microplowings were caused by traction of wear particles.Abrasive wear was main wear mechanism in the tests.Adhesive wear was rarely observed.Figure 4(b) shows the optical microscope image of friction surface on the Si 3 N 4 balls after wear

Figure 4 .
Figure 4. Optical micrographs of the friction surfaces; (a) raceway of the shot-peened ZrO 2 /SiC plate and (b) the Si 3 N 4 ball after 8 × 10 4 cycles at P mean = 967 MPa.Note that the dot-lined indicate the W wid .

Figure 5 Figure 5 .
Figure 5 shows the W vol values of ZrO 2 /SiC plates after 8 × 10 4 cycles.The W vol of the SP ZrO 2 /SiC plates clearly smaller than that of Non-SP plates.Figure 6 shows the W wid of the Si 3 N 4 balls after 8 × 10 4 cycles.The W wid values of the Si 3 N 4 balls used for tests on the SP plates were lower than those of balls used for tests on Non-SP plates.The aggressiveness to Si 3 N 4 balls was also reduced when

Figure 7
Figure 7 shows the friction coefficients of the SP and Non-SP plates during wear testing at P mean = 967 MPa.Each diamond symbol represents the average value of the friction coefficient during 2 × 10 4 reciprocation cycles.The average frictional coefficients of SP and Non-SP plates were almost identical at 0.65 and 0.63, respectively.Thus, the submicron-sized dimples on the surface of SP ZrO 2 / SiC plates shown in Figure 2(b) had little influence on the friction performance.

Figure 7 .
Figure 7. Friction coefficients of Non-SP and SP ZrO 2 /SiC plates in contact with an Si 3 N 4 ball at P mean = 967 MPa.

Figure 9 .
Figure 9. XRD measurements of the ZrO 2 /SiC plate surfaces; (a) Non-SP, (b) SP.Note that arrows are reference points.

Figure 11 Figure 10 .
Figure 11 illustrates the wear particle contact abrasion model.Micro-plowing occurred due to wear particle indentation and abrasion between the Si 3 N 4 balls and the ZrO 2 /SiC plates.The wear particles were formed by fractures from micro-sized radial cracks, micro-flaking, or micro-pitting in the near-surface of the plates.Continuous micro-flaking affects the increasing amount of wear of the ZrO 2 /SiC plate.When compressive residual stress is introduced into the near-surface of the SP ZrO 2 /SiC plates, radial cracks can be closed.Cracks are,

Figure 11 .
Figure 11.Improvement mechanism of sliding fatigue wear of the ZrO 2 /SiC plates under compressive residual stress.Note that radial cracks could be closed by compressive residual stress.

Table 1 .
Conditions of residual stress measurements.