By: P.P. Shukla and J. Lawrence
Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Leicestershire, LE11 3TU, United Kingdom.
Engineering ceramics offer a wide range of mechanical and thermal properties such as high melting point, corrosion and wear, bending strength, hardness, heat resistance under extreme conditions, physical stability, chemical inertness, very good electrical conductivity combined with their suitability for mass production. These properties are typically superior to conventionally used metals, plastics and glass. Fracture Toughness (K1c) is a measure of the materials resistance to fracture or crack propagation.
Materials with high K1c are much softer and ductile. Those types of material can resist cracks at higher stress levels and loading. Materials with low K1c are much harder, brittle and allow crack propagation at lower stresses and loading. Ceramics also do not mechanically yield as well as metals in comparison which leads to much lower resistance to fracture. Ceramics in comparison with metal and metal alloys have a low K1c, hence, it would be an advantage if the K1c of ceramics could be improved. This could open new avenues for ceramics to be applicable to high demanding applications where metals and metal alloys fail due to their low thermal resistance, co- efficient of friction, wear rate and hardness in comparison with ceramics. Surface treatment of silicon nitride and zirconia engineering ceramics with a CO2 laser and a fibre laser was conducted to identify changes in the fracture toughness. Vickers hardness indentation tests were employed prior to and after the laser treatment to investigate the near surface changes in the hardness of the engineering ceramics. Optical microscopy was then used to observe the near surface integrity, crack lengths and crack geometry within the engineering ceramics. A co-ordinate measuring machine was used to observe the diamond indentations and to measure the lengths of the cracks in the ceramics. Thereafter, computational and analytical methods were employed to determine the K1c.
Comparison of the laser treated surfaces with the as received surfaces showed that the K1c of silicon nitride was raised to 3.07 MPa m1/2 and 1.68 MPa m1/2 for zirconia ceramics when employing the CO2 laser. This was indicating that the CO2 laser reduced the hardness of the near surface layer which softened the ceramic and exhibited more resistance to fracture.
Laser treatment using the fibre laser increased the K1c by 1.8 MPa m1/2 with silicon nitride and 3.14 MPa m1/2 with zirconia ceramics when compared to the as received surfaces. In this case, the hardness of the fibre laser treated samples measured 4 % decrease with silicon nitride and 4% for zirconia. This meant that there was not a significant change in the hardness in comparison to the as received surface. However, the K1c was yet increased due to the reduction in the resulting crack lengths produced from the Vickers indentation test. The reduction in the crack lengths indicated that both ceramics treated by the fibre laser had become more resistive to crack propagation by a possible inducement of residual compressive stress. It can be concluded that the end effect of applying both laser types to the ceramics differ due to the differing wavelength which changed the thermal energy input into the ceramics, the brightness of the two lasers which could have possibly affected the absorption ratio and finally, the way in which the thermal heat is dissipated through the ceramics.
The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here