Study on the Relationship Between WC Coating Thickness and Residual Stress Using Critical Refracted Ultrasonic Longitudinal Waves

Tungsten carbide (WC) coatings of varying thicknesses were prepared using electrical discharge deposition technology. Relevant characterizations were conducted to analyze the residual stress in the WC coatings from a microscopic perspective, and this residual stress was measured using X-ray diffraction technology. Under isothermal conditions, a novel method for detecting the residual stress of the coatings utilizing critical refractive longitudinal (LCR) waves was employed to investigate the relationship between the residual stress of the WC coatings and their thickness. According to acoustic elastic theory, LCR stress measurement is based on the principle that stress within the material alters the propagation characteristics of ultrasonic waves. After correcting the effect of coating thickness on LCR propagation, the detection results of the LCR wave indicate that the compressive stress present in the coating may cause the substrate to exhibit a certain degree of tensile stress. At a coating thickness of 6–13 µm, as the thickness of the WC coating increases, the residual compressive stress within the coating gradually rises, leading to an increase in tensile stress on the substrate. However, at coating thicknesses of 13–16 µm, the changes in tensile stress on the substrate become minimal or even decrease, despite the continued increase in compressive stress within the WC coating. The relationship curve derived from the matrix surface aligns more closely with a quadratic function, while the curve obtained from the coating surface corresponds more to a linear function. This study employs LCR waves to detect residual stress in coatings, and the results indicate that LCR waves hold significant potential for application in the field of residual stress detection in coatings.