Sliding Wear Properties of Self-Mated Yttria-Stabilized Tetragonal Zirconia Ceramics in Cryogenic Environment Rohit Khanna and Bikramjit Basu w Laboratory for Advanced Ceramics, Department of Materials and Metallurgical Engineering, Indian Institute of Technology, IIT-Kanpur, India The objective of the present work is to investigate the friction and wear of self-mated ZrO 2 ceramics in a cryogenic environ- ment. Using a specially designed high-speed cryo-tribometer, fine-grained yttria-stabilized tetragonal ZrO 2 polycrystals (Y- TZP) were worn at varying loads (5–15 N) with sliding speed of 1.1 m/s in a cryogenic environment (liquid nitrogen, LN2). For comparison, the sliding tests were also conducted under selected operating conditions on self-mated Y-TZP under ambient con- ditions (room temperature (RT)), primarily to understand the difference in wear mechanisms for a given sliding condition. With these planned experiments, it was attempted to answer some important issues: (a) Can sliding in LN2 reduce the co- efficient of friction (COF) of self-mated ZrO 2 ? (b) Does t-ZrO 2 transformation occur in a cryogenic environment and if it occurs, how does it affect the fracture behavior? (c) How does the mechanism of wear change from RT to LN2 temperature? In our experiments, high COF (0.35–0.75) and high wear rate of disks 10 4 –10 6 mm 3 . (N . m) 1 have been measured under the selected tribological testing conditions. Interestingly, X-ray diffraction analysis revealed the presence of o-ZrO 2 after sliding in a cryogenic environment, while no change in phase assemblage was recorded after sliding under identical conditions at RT. An important observation has been that severe plastic deformation (wider and deeper grooves) at RT and microcracking (‘‘fish scale’’ pattern)-induced spalling of a damaged layer in an LN2 environment are the dominant wear mechanisms, respectively. I. Introduction I N recent times, there has been an increasing interest in using advanced structural ceramics for space applications, in par- ticular as turbopump bearings in space shuttle main engines (SSME). The space bearings typically operate under harsh con- ditions of high rotational speeds up to 50 000 rpm in pressurized cryogenic fluid, i.e. liquid nitrogen (LN2, 77 K) or liquid oxygen (LOX, 90 K) or liquid hydrogen (LH2, 20 K). 1–3 To date, now, SUS440C-grade martensitic stainless steel has been the most widely used material for the ball bearings in SSME, especially for the liquid rocket engine turbopumps. In general, for bulk materials like metallic alloys (SUS440C and 340 AISI stainless steel), engineering polymers (PTFE, PA6), and coatings (self- lubricating coatings, solid lubricant, diamond-like carbon), an extensive study has been reported on tribological behavior under lubricated and unlubricated test conditions and at cryogenic temperatures as well. 4,5–7 However, the high-speed sliding study on bulk ceramics at cryogenic temperature is rather limited. 1 Among the structural ceramics, ZrO 2 is one of the most im- portant ceramic materials, because of high fracture toughness and strength. Almost three decades back, CSIRO (Australia) scientists first demonstrated that zirconia-based ceramics can be toughened considerably by the martensitic tetragonal–mono- clinic transformation in localized transformation zones around cracks. 8 Since then, extensive research has been carried out on stabilization of ZrO 2 with oxides, such as Y 2 O 3 (Y-TZP), CeO 2 (Ce-TZP), MgO (MgO-PSZ), etc., and their composites. Phen- omenologically, the transformation toughening in zirconia ce- ramics is accompanied by subsequent volume dilation (4%–5%) during t-m ZrO 2 phase transition in the transformation zone around the crack tip. This helps in shielding the crack tip, there- by raising the energy required for crack propagation. 9 The relationship between the wear of the materials and their mechanical properties is an important issue in the field of tribo- logy. Fischer et al. 10 examined the wear behavior of self-mated zirconia, having varied fracture toughness. After conducting tribological experiments on stabilized ZrO 2 with tetragonal (K Ic : 11.6 MPa  m 1/2 ), mixed tetragonal and cubic (K Ic : 8.7 MPa  m 1/2 ), and the fully cubic phase (K Ic : 2.5 MPa  m 1/2 ), it was observed that the wear rate decreases with an increase in fracture toughness. A comprehensive study of the friction and wear of self-mated Y-TZP under different tribo-environments (air/water/paraffin oil/relative humidity) has been conducted in order to gain ex- perimental evidence of research being carried out so far and also to compare the present results with the literature. 10–19 A sum- mary of the related literature data is provided in Table I. Lubricated and unlubricated wear tests, 10–16 conducted on Y-TZP at room temperature (RT), revealed the friction coeffi- cient to be in range of 0.1–0.9, and the wear rate was found to vary from 10 7 to 10 1 mm 3  (N  m) 1 . This large variation in the coefficient of friction (COF) and wear rates is due to the strong influence of variation in the operating parameters, tribo- logical environment, and test configurations on the wear mech- anisms and the occurrence of wear transitions. Under a water- lubricated condition, the effect of sliding speed on wear is more significantly pronounced as compared with dry sliding. 15,17 Such an effect was initially described by the frictional heating-induced phase transformation during the wear of zirconia. Later, it was argued that for Y-stabilized zirconia, the yttria can be leached out, leading to destabilization of tetragonal zirconia in a water environment. Such an effect causes more wear in water, as com- pared with unlubricated sliding in an ambient environment. 15,17 In a different work, Fischer et al. 19 reported the wear behavior of Y-TZP under various environments (air/water/hexadecane). The worn surface topographical features indicated that the plas- tic deformation, accompanied by rolling of wear debris, was the predominant wear mode for Y-TZP under the RT testing con- ditions (air, 50% RH). Scott 14 reported the unlubricated wear behavior of Y-TZP as a function of temperature. It was ob- served that under the critical sliding conditions, the stresses and temperatures around the asperities at the tribocontact cause the Y-TZP material to undergo phase transformation from tetrag- onal to monoclinic phase. It needs to be mentioned here that no work, to the best of our knowledge, has reported the cryogenic wear of ZrO 2 . In our ongoing research to evaluate and understand the fun- damental mechanisms of tribological properties of bulk struc- R. Scattergood—contributing editor This work was financially supported by the Indian Space Research Organization (ISRO), Government of India, in designing and fabricating the cryogenic tribometer. w Author to whom correspondence should be addressed. e-mail: bikram@iitk.ac.in Manuscript No. 22841. Received February 24, 2007; approved April 23, 2007. J ournal J. Am. Ceram. Soc., 90 [8] 2525–2534 (2007) DOI: 10.1111/j.1551-2916.2007.01798.x r 2007 The American Ceramic Society 2525