Preparation methods of the TiCN coating
Since 1985, After Knotke first published research on the TiCN coating technology, People expressed great interest to its excellent high temperature oxidation resistance and good application performance, and so far had developed a variety of physical vapordeposition technology. At present, there are three methods of preparation of TiCN coating, which are magnetron sputter ion plating method, radiofrequency sputtering method and multi-arc ion plating method, in which the magnetron sputter ion plating method and multi-arc ion plating method are most widely used and low cost.
Magnetron sputter ion plating.
Magnetron sputtering technique was developed in the early 1970s, as the deepening of technology and research, it has been widely used in the industrialization field of electrical, optical film and energy, mechanical industrial etc. and become one of the most widely used preparation of TiCN film methods. In coating process, Ti ions are generated by using Ar ions generated by Ar gas glow discharge to bomb the Ti target, and by electrostatic acceleration fly to the work piece and thus deposition film. This method has high deposition rate, uniformity film thickness, and the ion plating can improve the combining ability of coating and substrate interface and make film organization dense. At the same time, the targets susceptible to pollution, and low deposition rate in the coating process are its main weakness. It has been found, when carbon and nitrogen partial pressure increases, the deposition rate will slow down.
Multi-arc ion plating.
Multi-arc ion plating belongs to an improved ion plating method, first developed by the Soviets, in the early 1980s, first practical by the U.S. Multi-Arc. The basic principle is taking the metal target source as the cathode, discharging by arc between the anode shell and making the target evaporation and ionization, forming space plasma, and then depositing coating on the work piece. Compared to other membrane technologies, the advantage is that the cathode produces plasma directly, and the cathode target can be arbitrarily arranged, which greatly simplifies the sample fixture. In addition, the multi-arc incident particle energy is high, the ionization rate can reach 60% ~ 80%,the density of membrane is high, the strength and durability are good, the interface of film and matrix is easy to produce atomic diffusion and the film adhesion is good.
Vacuum arc ion plating technique uses plasma electromagnetic field to filter, which can effectively reduce or eliminate large particles. Compared with conventional arc ion plating coating, arc-type filtered arc coating macro-particle is no impurities, homogeneous, dense structure, and able to meet the requirements of the optics, microelectronics film. There are also some disadvantages to the filtered arc source, that is, the beam diameter is small, usually less than 200nm, and difficult to form multi-arc source array, making the mass production of large area can not be achieved, and the transmission efficiency is not high, the maximum transmission efficiency of the bend structure approximately about 30%, the ion current just 2% to 3% of the arc current.
Influence of gas flow on the coating structure
Change of the N2 partial pressure (flow) will cause the sputtering nitrogen ion density and energy change, impacting the combining with the metal atom, making the preferred growth orientation change, thus affecting the coating performance. Reserachers found that under the condition of total pressure 0.8Pa and Ar flow 20sccm, when the nitrogen flow is less than 6sccm preferred orientation is (111), when the nitrogen flow is greater than 6sccm, (111) peak intensity decreases, and (200) peak intensity increases, mainly because in the structure of fcc-TiCN, (111) plane surface energy is low, under low nitrogen flow atoms are migration to (111) plane, with the increase of nitrogen flow, the atomic migration rate decreases, but (200) crystal surface with high surface energy has high step density, and the diffusion distance away from the low-energy grid points is short, benefit the crystal preferential growth along the (200) crystal surface. Reserachers found that when the nitrogen flow is 1sccm, samples obtained are amorphous structure, when the nitrogen flow is more than 2sccm, there is columnar structure in the film, grain boundary existing, when nitrogen flow increases to 6sccm, the film becomes dense, and respectively prefers to the microstructure of the isotropic and the grain refinement, mainly as the increase of nitrogen flow, the atomic migration rate is reduced, the membrane surface changes in local chemical potential. The researchers found that with nitrogen flow increasing, the grain gathered in the film is less, the surface becomes dense and smooth, the roughness decreases gradually until constant.
Now the carbon source used by the researchers’ preparation of TiCN is the C2H2 or CH4 gas mainly, because TiN and TiC are NaCl-type face-centered cubic structure, the radius of N atom and C atom is very close, N is 0.071nm, C is 0.077nm, the two can be mutually replacement to form a single-phase material TiC (N) or of TiN (C). Under certain condition, there may be two-phase structure appeared. In the XRD diffraction spectrum the peaks of them are very close, and even some overlap, resulting in the phase analysis complexity, so it is commonly written as TiCxN1-x.
Influence factors of TiCN coating performance
The quality of TiCN coating is mainly affected by the process factors like composition, temperature and atmosphere. Different matrix temperature will cause the coating’s grain size, shape, structure are completely different. The deposition temperature too high and deposition rate too fast will cause the coated crystal show thick branched, affecting the quality of the coating; the deposition temperature too low, it tends to form porous, loose sediments, affecting the binding strength of the coating and matrix. Therefore, a reasonable choice of temperature is a necessary condition to obtain high quality coating. Mc.Cormell etc. deposited TiCN coating on stainless steel with PVD method, including that its hardness, bond strength and coefficient of friction will not change when the temperature is below 250℃. After 450℃ heat treatment to samples, TiCN coating friction coefficient is 0.2 before 250℃,and up to 0.3 at 250℃, but still lower than the friction coefficient of TiN, it is because in TiCN coating C has played a lubricant role. Studies show that when the temperature is below 200 ° C, the coefficient of friction and wear rate of TiCN coating increases with increasing temperature.
The existence of pulsed bias plays a very important role to reduce the droplet and improve the coating quality. Negative bias attracting positive charge sputter work piece can make the titanium ions near the cathode target accelerate the fly, increase the chance of collision with nitrogen in the plasma and droplet, and at the same time increase the bonding strength of the titanium and nitrogen. If maintain the vacuum pressure constant, the nitrogen flow increases with increasing negative bias, but nitrogen content in the film decreases with the increasing of negative bias. This is mainly the Ti-Ti bonding ability is stronger than Ti-N, and with the increasing of negative bias, the re-sputtering ability of the titanium stronger than nitrogen. In addition, with the increasing of bias the plasma particles make the energy particles fly to the matrix change, affecting the organizational structure of the film.
Considered from the perspective of industrial production application, increasing arc current can improve productivity and film hardness and wear resistance. Increasing arc current, means that the target overall temperature increases, corresponding droplets will increase, and the droplet size will increase too.
Increasing of droplets and droplet size will inevitably lead to the decline of corrosion resistance of the film, especially large diameter droplets, with about 1/3 buried in the film in height direction and the irregular small holes at the bottom. When encounter corrosive substances like acid and alkali etc., these holes first corrupt and form needle-shaped holes, therefore, their existence is the main reason that the corrosion resistance of the coating reduces. Therefore, in practical application, for the coordination of the contradiction between increasing arc current and droplet, some optimized ways can be used, such as to increase the evaporation area of the target, strengthen the cooling effect of target or design new arc source which can inhibit droplets produce.