ORIGINAL ARTICLE Microstructure-property relation in alumina ceramics during post-annealing process after laser shock processing Fei Wang 1 | Chenfei Zhang 2 | Xueliang Yan 1 | Leimin Deng 2 | Yongfeng Lu 2 | Michael Nastasi 1,3,4 | Bai Cui 1,4 * 1 Department of Mechanical and Materials Engineering, University of Nebraska– Lincoln, Lincoln, Nebraska 2 Department of Electrical Engineering, University of Nebraska–Lincoln, Lincoln, Nebraska 3 Nebraska Center for Energy Sciences Research, University of Nebraska– Lincoln, Lincoln, Nebraska 4 Nebraska Center for Materials and Nanoscience, University of Nebraska– Lincoln, Lincoln, Nebraska Correspondence Bai Cui, Department of Mechanical and Materials Engineering, University of Nebraska–Lincoln, Lincoln, NE. Email: bcui3@unl.edu Funding information Division of Civil, Mechanical and Manufacturing Innovation, Grant/Award Number: CMMI-1563145 Abstract Laser shock processing (LSP) is a new surface engineering approach to introduce significant compressive residual stress into ceramics to improve their mechanical properties. However, LSP of ceramics may induce microcracks, which limit the further improvement of mechanical properties of ceramics. In this research, the effect of a post-LSP annealing process on a-Al 2 O 3 ceramics was investigated. The annealing treatment can cause thermal relaxation of compressive residual stress generated by LSP while still maintain the positive attribute of LSP. The compressive residual stress was stabilized after annealing after 10 hours at 1100- 1300°C. The healing of microcracks in a-Al 2 O 3 ceramics was observed during the post-LSP annealing process, which is caused by diffusion bonding mecha- nisms and accompanied by dislocation and void formation. The combination of the stabilized compressive residual stress and microcrack healing can improve the cracking resistance of a-Al 2 O 3 ceramics to mechanical impact on the surface by 69%. KEYWORDS annealing, laser shock processing, mechanical properties, microstructures, residual stress 1 | INTRODUCTION Laser shock processing (LSP) has attracted increased atten- tion as a new surface engineering approach to introduce significant compressive residual stress into ceramics to improve their mechanical properties. LSP utilizes high- energy nanosecond (ns) laser pulses to irradiate a sacrificial coating on the surface of ceramics to generate a plasma (Figure 1). A dielectric material transparent to the laser beam, such as water or glass, is used to confine the expan- sion of the plasma and increase its pressure level and dura- tion. 1 Consequently, laser-driven shock waves propagate into the bulk of the ceramics and create a biaxial compres- sive residual stress field. 2 The compressive residual stresses can extend to 1 mm or deeper below the surface. 2–4 The idea of using LSP to improve the mechanical properties of ceramics is motivated by literature that indicate the surface compression (e.g., by shot peening, thermal quenching, stress-induced phase transformation) can improve the mechanical properties of ceramics, such as fracture resis- tance, Weibull modulus, thermal shock resistance, wear resistance, and contact damage resistance. 5–8 The advan- tages of LSP compared to those traditional methods are deep penetration (>1 mm) of compressive residual stress, fast speed, precise control, accuracy, flexibility and no con- tamination. 9–12 For example, it is possible to obtain the desired LSP effect by precisely controlling the laser param- eters and accurately treat certain areas in the sample. In metallic materials such as stainless steels, Al alloys, and Ti alloys, LSP has been successfully used to improve *Member, American Ceramic Society. Received: 23 December 2017 | Accepted: 26 April 2018 DOI: 10.1111/jace.15742 J Am Ceram Soc. 2018;1–9. wileyonlinelibrary.com/journal/jace © 2018 The American Ceramic Society | 1