Growth and Properties of Diamond Films on Si3N4 Ceramic Substrates
To avoid failures such as cracking or elongation of silicon nitride(Si3N4),it is possible to use erosion-resistant diamond films.Diamond has very high hardness,high thermal conductivity,and a low friction factor,and its coefficient of thermal expansion is very close to that of Si3N4,which provides good adhesion,low residual stresses,and significantly increases the service life when deposited on Si3N4 substrates.Single-layer diamond films were deposited on Si3N4 substrates using hot filament chemical vapor deposition(HFCVD).A single variable control method was used to investigate the effect of the carbon source concentration,chamber pressure,and substrate temperature on the nucleation and growth of diamond on Si3N4 and to investigate the optimal parameters for the growth of micro-and nanodiamond films.Each of the three factors is defined as three series,where methane was selected as the carbon source.Series A represents the variation in the methane concentration,series B represents the variation in the chamber pressure,and series C represents the variation in the substrate temperature.The diamond surface and cross-sectional morphology,coating quality,and surface roughness of the diamonds in the as-deposited diamond films prepared with different parameters were characterized using Laser Raman Spectrometry(Raman),X-ray Diffraction(XRD),Scanning Electron Microscopy(SEM),and Atomic Force Microscopy(AFM).Indentation experiments were carried out using a Rockwell hardness tester to observe the cracks in the films and the area of delamination,in order to analyze the adhesion force between the diamond films deposited with different parameters and the substrates The results are summarized as follows:(1)The growth rate and surface roughness are mainly affected by the methane concentration:the growth rate increased from 0.84 μm/h to 1.32 μm/h when the methane concentration was increased from 1％to 7％,whereas the surface roughness(Ra)decreased from 53.4 nm to 23.5 nm.As the methane concentration increased,more carbon radicals were deposited in the films;however,secondary nucleation tended to occur and the non-diamond phase increased,resulting in a reduction in the surface roughness and film quality.Excessive methane concentration reduces the mechanical strength and hardness of the films themselves,decreasing the adhesion force with the substrate and increasing the delamination of the films when stressed by forces.(2)The chamber pressure affects the kinetic energy of the active material within the reaction chamber that reaches the substrate.As the pressure increased,the kinetic energy decreased,which is unfavorable for diamond nucleation and growth.Although the diamond content was highest in the films grown at 2 kPa,analysis of the other factors collectively showed that the adhesive force for the substrate and surface morphology of the films grown at 2 kPa were not as good as those of the diamond films deposited at 1 kPa.(3)The surface morphology and diamond phase composition of the films were significantly influenced by the substrate temperature.At 800 ℃,the surface of the coating could not form complete and continuous diamond crystals,many cavities appeared and no obvious diamond characteristic peaks appeared in the Raman spectra.Therefore,a temperature below 800 ℃ is not suitable for the growth of diamond.When the temperature was increased from 850 ℃ to 900 ℃,the nucleation density of the diamond surface and the quality increased.In addition,the content of the non-diamond phase was reduced,and the surface had a more stable(111)crystalline structure,which enables the growth of high-quality films and provides a stronger adhesion force to the substrate.(4)The optimum growth temperature and air pressure were 900 ℃ and 1 kPa,respectively.The methane concentration in the reaction chamber was adjusted to control the diamond grain size and sp2 carbon content,giving films with 1％microdiamond and 5％nanodiamond.The preparation parameters for diamond films with excellent properties on ceramic substrates were optimized using three-factor-coupled experiments.
diamond filmssilicon nitridehot filament chemical vapor deposition(HFCVD)
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