ICMCTF2016 Session B4-3: Properties and Characterization of Hard Coatings and Surfaces

In cutting operations, wear and tool failure are often a result of the high temperatures occurring during application. Despite this fact, typically the mechanical properties of hard coatings are considered as key-parameter to estimate their performance. The influence of the thermo-physical properties is widely neglected, although they significantly affect the local conditions and thus, the application behaviour. The thermal expansion coefficient has for example a major influence on the evolution of residual stresses within a coating, while the thermal conductivity determines whether the coating provides a functional thermal barrier to protect the tool and a high heat capacity can intentionally provide heat sinks. For hard coatings, the thermo-physical properties can vary in a wide range depending on the chemical composition, observed microstructure and applied post treatments like e.g. thermal annealing. Further, tabular data is barely available which necessitates determination of thermo-physical properties to illuminate their influence on the coating performance. Within this work, high-temperature X-ray diffraction and wafer curvature experiments were performed up to 700 °C to determine the thermal expansion coefficients of TiN, AlN and TiAlN hard coatings. The results obtained by both methods are compared and critically discussed. Further, the thermal conductivity of as-deposited and post treated TiAlN coatings was investigated at room temperature using time-domain thermoreflectance measurements. The obtained values range between 2 and 20 W/mK, depending on the coating microstructure. For the investigation of the specific heat capacity, differential scanning calorimetry was successfully applied up to 700 °C, indicating excellent agreement of TiN and AlN with values provided by NIST and a reasonable conformity of TiAlN to values estimated by the Neumann-Kopp rule.

Three grades of virgin perfluoroalkoxy (PFA) powders were post-blended with four types of additive powders (graphite, silicon carbide, alumina and boron nitride) to modify the thermal, mechanical and tribological properties. Disk samples were prepared by compression molding for thermal property characterizations, i.e., laser flash to measure thermal diffusivity, and differential scanning calorimetry to measure specific heat. The composite powders were electrostatically applied to stainless steel substrates, and the mechanical and tribological properties were characterized by means of instrumented microindentation, microscratch, and ball-on-disk wear tests. The thermal conductivity of the PFA can be increased by a factor of 2 to 3.5 by adding 20% (weight percentage) of graphite or boron nitride powder, while the addition of the same amount of silicon carbide can only moderately increase the thermal conductivity by about 40~60%. Adding 20% alumina essentially does not change the thermal properties of the PFA. Examination of surface deformation behaviour and failure mechanisms revealed that the surface hardness, the roughness and the extent of plastic deformation mutually affected the wear resistance of the polymer composites. As such, graphite-filled composites that had the lowest level of plastic deformation and the highest hardness, showed the lowest critical load for coating failure and poor adhesion to the substrate. The incorporation of boron nitride, silicon carbide and alumina particles in PFA induced similar hardness and deformability, however, the critical failure load for alumina-filled coating was at least 22% higher. This was attributed to the superior performance of alumina particles in maintaining the viscoelastic properties of the polymer matrix and the lower surface roughness of alumina-filled coatings. The three grades of PFA also showed different thermal and tribological properties after adding the same reinforcement particles.

SiBCN alloys are known [1-4] - depending on the elemental composition - for their thermal stability and high-temperature (up to 1500°C ) oxidation resistance, hardness, optical transparency or electrical and/or thermal conductivity. This contribution deals with ageing of a very wide range of SiBCN coatings prepared by reactive magnetron sputtering of Si_x(B₄C)_1-x targets in N₂+Ar mixtures. Specifically, attention will be paid to the long-time room-temperature oxidation resistance, expressed in terms of the thickness and properties of the surface oxide layer (characterized by spectroscopic ellipsometry) 12 years since the deposition [1].

On the one hand, two subsets of coatings which exhibit (at least at some substrate bias, U_b) perfect long-time room-temperature oxidation resistance have been indentified (this includes those, but not only those, which exhibit short-time high-temperature oxidation resistance): either (i) high Si content or medium Si content and Ar-rich gas mixture or (ii) very low Si content and Ar-poor gas mixture. On the other hand, between these two subsets there is a strip of elemental compositions and preparation conditions leading to formation of a surface oxide layer, or even to complete oxidation down to a substrate. The Si-poor films prepared at a low or medium Ar fraction in the discharge gas mixture exhibit relatively better performance at the high |U_b| of 500 V, while the other films exhibit relatively better performance at the low |U_b| of 100 V. There is an excellent consistence of the ageing resistance studied and the densification (expressed in terms of film hardness and refractive index). The surface oxide layer is significantly thicker on films stored under the open air than on films stored in polyethylene bags (protected against e.g. water vapor).

Collectively, the results constitute detailed ageing resistance maps which provide an insight into the complex relationships between the elemental composition, preparation conditions and materials performance, and which consequently allow one to design long-lifetime SiBCN coatings for a wide range of technological applications.

[1] J. Houska, Ceram. Int. 41, 7921 (2015)

[2] J. Vlcek et al., Surf. Coat. Technol. 226, 34 (2013)

[3] V. Petrman et al., Acta Materialia 59, 2341 (2011)

[4] J. Houska et al., Europhys. Lett. 76, 512 (2006)

ZrN thin films were deposited on AISI 304 stainless steel (SS) by hollow cathode discharge ion-plating (HCD-IP) and unbalanced magnetron sputtering (UBMS). Afterwards, the specimens were annealed at 1000 °C in vacuum (4×10^-6 Torr) for a duration ranging from 1 to 4 hr. The purpose of this study was to investigate the oxidation behavior and corrosion resistance of the vacuum annealed ZrN-coated SS prepared by the two deposition methods. The X-ray diffraction (XRD) results indicated that ZrN remained as the major phase in the oxidized thin films even after heat treating at 1000 °C for 4 hr in vacuum. Since the grain size of the as-deposited ZrN films by UBMS was larger than that by HCD-IP, the surface grain size of the oxidized film showed the similar trend, which also affected the phase formation after vacuum annealing. The oxidized ZrN thin film on 304SS formed two oxide layers, the outer layer on the film surface and the inner layer nearby the film/substrate interface, which could be attributed to the simultaneous diffusion of oxygen from film surface and the edge of film/substrate interface, respectively. The results of potentiodynamic polarization scan showed that the corrosion resistance of the oxidized ZrN-coated SS was excellent and increased by about 1000 times relative to that of bare 304SS. The corrosion area after 500-hr salt spray test was less than 6.7 % for the oxidized ZrN-coated SS, and the oxidized ZrN films deposited by UBMS had better durability in salt spray test. The fully oxidized ZrN thin film deposited by UBMS showed great corrosion resistance and remained intact on 304SS substrate after potentiodynamic polarization scan, which was associated with the enhanced adhesion by the interdiffusion layer from heat treatment. The results indicated that ZrN thin films deposited by both UBMS and HCD-IP could be used for the growth of oxidized layer by vacuum annealing. Since HCD-IP is not a popular technique as UBMS, UBMS can replace HCD-IP to deposit ZrN thin films on 304SS to produce an oxidized ZrN layer with excellent corrosion resistance.

Effect of scanning speed on titanium aluminide-Ti₃Al produced in-situ during laser metal deposited TiC/Ti6Al4V has been investigated and its effect on microhardness and wear resistance properties has been studied. In this study, titanium alloy –Ti6Al4V (an important aerospace alloy) was deposited in combination with titanium carbide-TiC using laser metal deposition process. The laser power was maintained at 3.2 kW throughout the deposition process. The powder flow rate and the gas flow rate were also kept at constant values of 2.88 g/min and 2 l/min respectively. The scanning speed was varied between 0.015 and 0.105 m/s , the influence of the scanning speed on the titanium aluminide (Ti₃Al) produced In-situ was studied and its effect on the wear resistance behaviour was investigated. The study revealed that as the scanning speed was initially increased, the Ti₃Al produced In-situ was found to increase and the wear resistance was found to improve. As the scanning speed was further increased beyond 0.06 m/s the Ti₃Al produced was found to decrease so also the wear resistance property was found to decrease.

Surface enhancement of engineering materials is necessary for preventing service failure and corrosion attack in the industries. Surface hardening of mild steel with Zn-Sn bath is characterized by the traditional phases, whereas presence of Al leads to coatings constituted by phase in the inner layers, and by an outer layer made of three phases and intermetallic phases which made the materials to be brittle. The investigation of Zn-Al-Sn coatings on AISI 1015 steel by laser alloying technique is aimed at enhancing the properties of Zn-Al-Sn coatings on AISI 1015 steel. The laser power of 750W, scanning speed of 0.6 and 0.8m/min, alloy composition of 25%Zn-50%Al-25%Sn and 30%Zn-40%Al-20%Sn were used in the research. A 4.4kW continuous wave Rofin Sinar Nd:YAG laser was utilized for the fabrication process. The steel alloyed surfaces were investigated for its wear, hardness and corrosion behaviour at different laser processing conditions. The steel samples were cut to corrosion coupons, and immersed into 0.5 M H₂SO₄ media at 28⁰C using electrochemical technique. The microstructures of the developed intermetallic coatings and uncoated samples were characterized by optical (OM) and scanning electron microscope (SEM/EDS). Furthermore, X-ray diffractometer (XRD) was used to identify the phases present. The results showed improved properties by increasing the Al content from40 to 50%. The optimum properties was obtained at 25%Zn-50%Al-25%Sn at laser power of 750W and speed of 0.8m/min.The optimum composition gave protection efficiency of 99.88% in 0.5 M H₂SO₄ solution and 2.2times the hardness of the substrate were achieved. The improved hardness was attributed to the hard intermetallic phases (Al Fe, Al_0.71 Zn_0.29, Fe Zn_6.67, Fe Sn₂, Al₁₃ Fe₄). It has been established that laser alloying of AISI 1015 steel with of Zn-Al-Sn is promising for improving the surface hardness values and corrosion resistance.