Evaluation Technique Of Mechanical Properties For Ceramic Films Or Coatings

Due to the excellent high temperature-, corrosion-, wear- and oxidation resistance, ceramic films or coatings are widely used in many fields, such as aerospace, automotive, textile engineering and so on. Many of their applications are often related to mechanical properties. Therefore, evaluating mechanical properties of ceramic films or coatings plays an important role in engineering applications. And the main objectives of this thesis are to study the mechanical properties of ceramic films or coatings, such as hardness, elastic modulus, fracture toughness, interfacial bonding strength, impact resistance, scratch resistance, thermal shock resistance and so on. The relative descriptions are as follows:1. High speed steel substrate / nitride coating was used as the specimen. The absolute hardness of the coating was analyzed by J-H (J(o|¨)nsson and Hogmark) and L-C (Lesage and Chicot) models. A new model was established to evaluate the absolute hardness of the coating. The variable trend of the absolute hardness of the coating under different loads obtained by the new model is identical to that obtained by J-H model and L-C model. The absolute hardness of the coating calculated by the new model was 19.8±2.3 GPa. The micro-cracks and crushes near the Vickers indent may be related to the variations of the calculated absolute hardness under different loads.2. The hardness and elastic modulus of common glass and stainless steel were calculated by the depth-sensing indentation method. The results were close to that tested by traditional method or reference value, indicating that the depth-sensing indentation method was reliable. The hardness and elastic modulus of CVD SiC hard coating obtained by this method were 37.7 GPa and 456.4 GPa, respectively. The mean value of the fracture toughness K_IC of the coating obtained by three different fracture toughness equations was 2.70 MPam^1/2. The value was a little lower than the reference fracture toughness value of bulk silicon carbide. 3. The interfacial bonding strength of silicon substrate / diamond-like carbon (DLC) coating was tested by indentation and cross-bonded methods. The critical loads of Vickers and Hertz indentation for the samples were 0.981 N and 300 N, respectively. For Vickers indentation, a circular damage was initiated on the surface of the coatings when the applied load was higher than the critical load. When the applied load was further increased, radial cracks would appear in the coating. For Hertz indentation, the number of the ring cracks increased from 1 to 4 with increasing the load from 300 N to 800 N. The interfacial tensile and the shear bonding strengths of the silicon substrate / DLC coating calculated by the cross-bonded method were (8.9±2.7) MPa and (20.1±2.6) MPa, respectively.4. Steel substrate / ZrO₂ coating and aluminum substrate / ZrO₂ coating fabricated by thermal spraying technique were used as the specimens. Spherical impact indentation method with different impact angles (90°, 60°, 45°, and 30°) was applied to understand the impact resistance of the specimens. The variable trends of the impact force and impulse for the specimens under the same and different impact angles were investigated. Based on the impact waveforms and surface damage morphologies, it can be predicted that ZrO₂ coating on steel substrate exhibits higher impact resistance than that on aluminum substrate.5. Scratch resistance for different substrate-coating systems was studied by scratch method. As for silicon substrate / silicon carbide coating, the critical load for this coating decreased with increasing maximum normal force and responding loading rate under the same scratch length. The scratch morphologies were also investigated. And the differences of acoustic emission curves and morphologies for different substrate-coating systems under the same scratch condition were studied. It is demonstrated that the critical load for hard-brittle coating can be effectively obtained by scratch method.6. Steel substrate / ZrO₂ coating and aluminum substrate / ZrO₂ coating were used to carry out the water quenching experiments. According to morphologies of the coating surface and cross section of the substrate-coating system after water quenching, it can be observed whether spallation or micro-cracks appeared during the thermal shock process. As for steel substrate / ZrO₂ coating, when the temperature difference during the water quenching was above 800°C, a small amount of micro-cracks can be observed in the coating. However, no apparent change can be investigated for the substrate-coating system, indicating that ZrO₂ coating has excellent quenching thermal shock resistance. Based on the local heating method under petrol and oxygen atmospheres, it is demonstrated that ZrO₂ coating on steel substrate exhibits excellent rapid-heating thermal shock resistance and the conditions ZrO₂ coating surface on steel substrate can resist are less than 8s for fire-jet time and less than 1450°C for fire-jet temperature.