Fracture toughness modification by using a fibre laser surface treatment of a silicon nitride engineering ceramic

Surface treatment of a silicon nitride (Si₃N₄) engineering ceramic with fibre laser radiation was conducted to identify changes in the fracture toughness as measured by K_1c. A Vickers macro-hardness indentation method was adopted to determine the K_1c of the Si₃N₄ before and after fibre laser surface treatment. Optical and a scanning electron microscopy (SEM), a co-ordinate measuring machine and a focus variation technique were used to observe and measure the dimensions of the Vickers indentation, the resulting crack lengths, as well as the crack geometry within the as-received and fibre laser-treated Si₃N₄. Thereafter, computational and analytical methods were employed to determine the K_1c using various empirical equations. The equation K_1c = 0.016 (E/Hv)^1/2 (P/c^3/2) produced most accurate results in generating K_1c values within the range from 4 to 6 MPa m^1/2. From this it was found that the indentation load, hardness, along with the resulting crack lengths in particular, were the most influential parameters within the K_1c equation used. An increase in the near surface hardness of 4% was found with the Si₃N₄ in comparison with the as-received surface, which meant that the fibre laser-treated surface of the Si₃N₄ became harder and more brittle, indicating that the surface was more prone to cracking after the fibre laser treatment. Yet, the resulting crack lengths from the Vickers indentation tests were reduced by 37% for the Si₃N₄ which in turn led to increase in the K_1c by 47% in comparison with the as-received surface. It is postulated that the fibre laser treatment induced a compressive stress layer by gaining an increase in the dislocation movement during elevated temperatures from the fibre laser surface processing. This inherently increased the compressive stress within the Si₃N₄ and minimized the crack propagation during the Vickers indentation test, which led to the fibre laser-radiated surface of the Si₃N₄ engineering ceramic to have more resistance to crack propagation.