Study Of Composite Hard Films Prepared By MF Unbalanced Magnetron Sputtering

The thin film technology is advancing the diversified, gradient and composite direction at present. Preparing multi-layer composite coating, and optimizing the deposition process considering properties of composite element and relevance of deposition conditions, is the effective means to improve coatings'mechanical properties. Under these circumstances, it is necessary to study on different types of Ti (C, N), (Ti, Al) N, DLC hard coatings and various multilayer composite coating preparation technology, which has greater practical significance for expanding the application of such hard coatings.Hard film has a good prospect, but traditional hard film preparation process is mainly arc ion plating, PCVD and DC reactive magnetron sputtering technology, which all have some disadvantages, for example, film prepared by arc ion plating technology contains droplets, and the surface roughness is great; while DC reactive magnetron sputtering has such problems as cathode target poisoning, low ionization rate, sputtering efficiency and anode disappearing which leads to instability coating process, bad binding force between film and substrate, PCVD exhaust pollution, and so on.Supported by National High Technology Research and Development Program of China (863 Program) (No.2001AA338010) funding, different film systems of non-hydrogen DLC(α-C), hydrogen DLC(α-C:H), nitrogen-doped DLC(α-C:N,α-C:H:N), Ti(C,N) series and different Al content (Ti, Al) N hard composite film are fabricated on different materials with multifunctional infiltration composite coating equipment by ion beam assisted deposition technology combined with MF twin target sputtering and unbalanced magnetron sputtering technology.Using SEM, XRD, Laser Raman spectrometer, XPS, Micro-nano mechanical testing instrument (CMS), Inconsistent stylus profilometer, micro-hardness tester, Triometer tester, spectrophotometer, etc., the analysis of those hard composite film samples are conducted. The research is focused on influence of the structure and fabricated parameters (such as, the working atmosphere (flow) and the doping amount, substrate bias, substrate temperature and Hall ion current, etc.) of multilayer gradient composite film like Ti(C,N), TiAlN and DLC film (for example, the influence of different middle layer in composite film on the overall adhesion and performance) on the properties of thin films (such as, film structure, surface color, roughness, friction coefficient, hardness and film-substrate binding force, etc.). It provides the experimental basis for the industrial application of multilayer hard composite film.The experimental results and the conclusions of the analysis are listed as the following:(1) Using the same composite coating equipment, the Ti(C,N) series composite film, non-hydrogen DLC(α-C) composite film, hydrogen DLC(α-C:H) composite film, nitrogen-doped DLC composite film and Ti1-xAlxN composite film, etc., are prepared with ion beam assisted MF unbalanced magnetron sputtering and reactive sputtering process by changing the target material and the connection of target power. It resolves the defects of film by other traditional method, such as rough surface, large internal stress, and the hardness, adhesion strength and other properties of films are maintained.(2) The Al content of (Ti, Al) N film has great influence on film properties. At present, domestic and international arc ion plating and magnetron sputtering process are based on titanium aluminum alloy target to fabricate (Ti,Al) N film. However, the high Al content of TiAl alloy makes the alloy hard and brittle, so the preparation of TiAl alloy target with high Al content is very difficult, and the price of TiAl alloy target is relatively high. At the same time, the titanium and aluminum content of target have been fixed during preparation, can not arbitrarily change. In this paper, MF twin-targets unbalanced magnetron sputtering technology is used to prepare (Ti, Al) N film. The target material used is separate Al and Ti. It can not only solve the difficulties of TiAl alloy target on fabrication, but also easily control chemical composition of thin film during deposition processing by adjusting the power (current) of MF target. Using this technology, (Ti,Al) N film with high Al content can be prepared, and preparation cost can be greatly decreased.(3) Combining the respective merits of arc ion plating and magnetron sputtering, the arc ion plating and unbalanced magnetron sputtering are synthetically used in the same coating equipment. It further expands the scope of the application of magnetron sputtering. Taking into account its high ionization rate, arc ion plating process is used in ion bombardment cleaning and Cr basement coating preparing, while MF unbalanced magnetron sputtering process is used in the other coatings'deposition. The measure gives full play to their respective advantages of both technologies. Better binding force between substrate and film can be gotten, and the droplet by arc discharge is effectively reduced in magnetron sputtering process, so the deposited films are dense and smooth.(4) Ti(C,N)/DLC and (Ti,Al)N/DLC hard composite film series are prepared. Self-lubricating of DLC films and good mechanical properties of Ti(C, N), (Ti, Al)N films are combined to improve their comprehensive performance.(5) In order to change the high internal stress and poor attachment of DLC film, different types of multi-layer gradient composite DLC film system are designed and prepared, such as Ti/TiN/Ti(C,N)/TiC/DLC, Cr/Ti/TiN/Ti(C,N)/DLC and Cr/(Ti,Al)/(Ti,Al)N/DLC, which can ensure that hardness of films increase gradually from the inside to the outside, and lower the film stress and difference of thermal expansion coefficient between layers. The XPS analysis results on the composite film confirm that, the design can effectively improve the mechanical condition of composite film. The multi-layer gradient composite film system whose composition and structure change gradually between DLC film and substrate, makes higher residual stress in DLC film greatly reduced and enhance the film's binding force and mechanical properties.Cr is chosen as the basement coating of the transition layers in Ti (C, N)/DLC and (Ti, Al)N/DLC composite film systems, considering that the adhesion is strong between Cr and steel, the use of Cr can effectively improve the interface bonding force. XPS analysis confirm that, a smaller concentration gradient Cr-substrate interface layer forms when Cr film is bombarded by high-energy ion during film deposition process, which reinforce the combination intensity with steel substrate. Ti, Al mixed film is prepared after Cr film deposition. Concentration gradient of Ti and Al rise and fall respectively, being a mixing zone of Ti, Al, N ingredients, so the concentration change gradually, thus reducing the internal stress in film to a large extent.(6) The suitable process of Ti(C,N) series, non-hydrogen DLC(α-C), hydrogen DLC (a-C:H), and Ti1-xAlxN hard composite films by MF unbalanced magnetron sputtering and reactive sputtering technology are concluded by experiment.(7) The crystal structure and performance of DLC film are greatly effected by substrate temperature. When substrate temperature is 80℃～120℃, the sp3 bond content of DLC film is the largest, and the film's hardness and Young's modulus also reaches the largest. While when the temperature continues to increase, the hardness and Young's modulus decline due to the sp3 content decrease sharply in the films.(8) DLC film is liable to form at lower substrate bias, while at a higher bias, the ion bombardment enhances, so the temperature on film surface increases and film tends to graphitization. When the substrate bias is appropriate, the ion bombardment energy is moderate, and sp3 bond content in DLC film reach the largest, film hardness and Young's modulus also reach the maximum.When the substrate bias is larger, the inverse sputtering effect of high-energy particles on the film enhances and cause surface damage, which leads to DLC film's surface roughness increases. When substrate bias is in certain range, with the increase of Hall discharge current, ion beam bombardment density on the film surface increases, so the fragile deposition particles on surface will strip off, and roughness decreases, reflectivity increases, color becomes bright gradually for DLC film.Substrate bias influences the preferred orientation of Ti(C,N) film's crystal structure, when substrate bias is larger, the film show more obvious preferred orientation.(9) Hall ion source discharge current has larger influence on the structure and properties of film. With the increment of Hall discharging current, the thermodynamic steady graphite structure is liable to form because the bombardment of energetic particles on growing film results in the active shift of carbon atoms. In the meantime, the sp2 content increase, and the sp3 content, hardness and Young's modulus all decrease gradually.With the increase of the Hall discharge current, the film becomes smooth and the friction coefficient decline, which can be explained that the inverse sputtering of particles on the film surface is strengthened because of the enhanced current, and the atoms supplied with energy can shift on film surface to improve the film's compaction as a result. Hall ion assisted deposition can effectively enhance the reflectivity, bright color and increase film's bonding force.(10) The substrate material has great influence on film's mechanical performance, especially hardness and binding energy. The testing hardness, failure mode and binding force of the same hard film on different hard substrate are all different. The mechanical performance of composite films with different transition layers on the same substrate is also different. The closer the characters of film and substrate material are, the more remarkable the surface modification effect of film is.(11) When depositing Ti(C,N) film, its hardness decreases with the increase of acetylene-nitrogen ratio. The nitrogen-acetylene ratio should be greater than 3:2. The color of TiN coating is sensitive to the working atmosphere. The unbalanced magnetic field can increase the plasma concentrations in space, but when the plasma density is relatively little, small change of plasma density during magnetron sputtering process will cause considerable change in the coating's color. With the increase of acetylene inlet flow, Ti/TiN/Ti(C,N)/TiC film's absorption of various band light increases, and the reflectivity reduces. The color of film turns black. When the acetylene inlet flow increases to 300 sccm, the film will turn to a preferred (111) orientation from non-preferred orientation.When depositing nitrogen-doped a-C:N film, with the increase of N2 content in reaction gas, the deposition rate of a-C:N film gradually increases, and the film's surface roughness increases. The flow ratio of Ar/N2 has influence on the film structure and mechanical properties. When Ar/N2 is 4.245:1, diamond phase sp3 bond content in the a-C:N thin film is the largest, the hardness and Young's modulus of film are also the largest,17.151 GPa and 179.838GPa, respectively. With the continual increase of N2 ratio, film hardness and Young's modulus reduce because of the forming of lots of carbon-nitrogen bond, when the color of a-C:N film mainly takes on black. With the increase of N2, the film color turns red gradually.When depositing a-C:H film, with the gradual increase of C2H2, C ion increases, so the a-C:H film deposition rate correspondingly increases. At the same time, the film surface becomes rough as a result of targets poisoning and impurities on targets being sputtered to the film. With the increase of C2H2, the color of a-C:H film gradually becomes black, the self-lubricating of a-C:H film enhances and the friction coefficient decreases. C2H2 flow has little influence on film's structure.When doping N2 inα-C:H film, with the increase of N2 flow, the sp3 bond content inα-C:H:N film gradually increases. When N2 flow is 30 sccm, the sp3 bond content in film reaches the biggest. After the nitrogen is doped, the color of a-C:H:N film takes on black with slightly red. With the increase of doped nitrogen content, the friction coefficient of a-C:H:N film shows a decreasing trend, so nitrogen-doped can improve the film's tribological properties. (12) The corrosion resistance of substrate plated with DLC film is significantly improved, for DLC film replaces the oxide layer. The microscope observation and potentiodynamic canning result have indicated that the DLC film have good corrosion resistance.(13) The Al content has great influence on the crystal structure, hardness and mechanical properties of composite Ti1-xAlxN film. When the Al content increases, the crystal structure of (Ti,Al)N film change from cubic crystal structure to hexagonal AlN structure, and the (111) preferred growth turns to (200) and (220) preferred orientation.With the increase of Al content, the hardness of Ti1-xAlxN film firstly increases and then decreases. When the Al content in Ti1-xAlxN film equals 51 at%, the hardness and film-substrate binding force is the largest. When the critical load Lc2 is 50N, hardness of film deposited on high-speed steel is 2078HV, which is slightly greater than the (Ti,Al)N film prepared by magnetic filtered cathodic vacuum arc ion plating process when the substrate material is same.Suitable Al addition can reduce the friction coefficient of Ti1-xAlxN film. When the Al content is less than certain value, the friction coefficient of film decreases with the increase of Al content, the minimum value reaches 0.14 when x equals to 51 at%, and then turns greater rapidly with the increasing of Al content.(14) Compared with the film prepared by arc ion plating, the film prepared by MF unbalanced magnetron reactive sputtering is much smoother, and the color is close to that by arc ion plating, but brighter; the hardness and binding strength of film prepared by MF unbalanced magnetron sputtering and arc ion plating are almost the same.