Residual surface stress by localized contact-creep
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Residual surface stress by localized contact-creep Sujanto Widjaja, Karl Jakus,a) Revti Atri, John E. Ritter, and Sandeepan Bhattacharya Department of Mechanical Engineering, University of Massachusetts, Amherst, Massachusetts 01003-2210 (Received 6 November 1995; accepted 27 June 1996)
When a ceramic material creeps under a localized stress and then cools under load, a portion of the creep flow stress is retained as a residual compressive stress due to elastic rebound being constrained by the creep zone. Localized contact-creep was used to generate residual compressive surface stress in soda-lime glass and two sintered aluminas. The Vickers indentation technique was used to measure the residual stress within the contact-creep area. Alumina with a higher elastic modulus than glass retained higher residual compressive surface stress. The results were in reasonable agreement with the predicted stress distribution given by finite element analysis.
I. INTRODUCTION
Ceramics are of increasing interest for engineering applications because of their high hardness and resistance to corrosion and wear. Moreover, their low thermal conductivity and resistance to high temperature make them well suited for heat shields, engine valves, bearings and seals, and heat exchangers. On the other hand, ceramics are brittle and hence sensitive to tensile and impact loadings and thermal shock. Therefore, it is important to design ceramic materials that have high strength and toughness. A residual compressive stress layer at the surface is commonly used to increase the strength and damage tolerance of glasses and ceramics. In glass surface compressive stresses are generated by either a thermal treatment or ion-exchange. Residual compressive stresses can also be introduced into ceramic components. Zirconia-toughened ceramics (ZTC) can be fabricated with a surface compressive stress by inducing the tetragonal ZrO2 to transform to the monoclinic phase in the surface.1–4 Also, thermal expansion mismatch has been used in SiC –AlN layered composites to produce residual compressive stress in the surface.5 However, all these techniques are limited in applicability since they are specific to the material system being exploited. In practical applications, it is common to find design of ceramic components where stress concentration at a notch or fillet region is unavoidable, making these areas of stress concentration susceptible to premature failure. Thus, it is desirable to be able to strengthen ceramic components in these highly stressed locations. One such means would be to generate localized residual
Presented at the 97th Annual Meeting of the American Ceramic Society 1995, Cincinnati, OH (Functionally Graded Materials Symposium, Paper No. SXII-2-95). a) Address all correspondence to this author. 210
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J. Mater. Res., Vol. 12, No. 1, Jan 1997
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compressive stress in these critical areas and, therefore, improving the strength and reliability of the
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