TiN,CrN,TiN/CrN Films Coated With Arc-Glow Penetrating Technique And Their Properties

Magnesium is the lightest metal used in engineering. Magnesium alloys have advantageous properties including high specific strength, high specific toughness, low elastic module, excellent damping characteristics, good machinability and dimensional stability, electromagnetic shielding, abundant in resource as well as easy to be recycled. So magnesium alloys have found a wide applications in such areas as automobile, computer, electronics, aircraft and astronautics, sporting goods, handhold tools and household equipment, et al., and been regarded as "green engineering material" with great potential development in 21^st century.However, there are also some unsatisfactory characteristics of the metal. Among these drawbacks, inferior tribological behavior and corrosion problem are especially fatal that have restricted the actual use of magnesium alloys. Poor wear resistance results in the use of magnesium alloys is until yet limited to static components. Also, these alloys are extremely susceptible to galvanic corrosion due to the much lower electrode potential of magnesium.Corrosion and wear are surface damages of materials. So an easy and effective method to improve wear resistance and corrosion resistance of magnesium alloys is surface enhancing. Recently, a number of surface processes have been developed, such as electroplating, electroless plating, anodizing, plasma micro-arc oxidizing, vapor depositing, conversion coatings, organic coating, and et al. Those coatings do protect magnesium alloys in some ways. But the limitations are also distinctive.To meet the requirement of developing new surface treatment of magnesium alloys, TiN,CrN mono-layer films and TiN/CrN multi-layer film had been coated on magnesium alloy AZ91D by arc-glow plasma penetrating technique. The aim of the study is to improve the wear resistance of the magnesium alloy, supporting a much wider uses of this material.Arc-glow plasma penetrating is a novel surface coating technique that has been developed by the Research Institute of Surface Engineering of Taiyuan University of Technology. Based on Double Glow Plasma Surface Alloying, this process adds one or more cold arc cathodes as alloying elements supplication. In the beginning, the substrate was cleaned, activated and heated by glow discharge. Then the arc sources were ignited and continually emitted ion beams of coating metals with high energy, high current density and high ionizing ratio. With the help of negative bias, those ions quickly move to the surface of substrate and deposited on. As the ion bombard working on, the surface of the parts has formed deposited layer, penetrated layer and hybrid layer. The coatings may be also nitride ceramic, if N₂ was introduced during the course of deposition. The advantages of Arc-glow plasma depositing include:â‘ deposited quickly,â‘¡robust adhesion between coating and substrate,â‘¢dense and homogeneous features of coating,â‘£less investment on equipment,⑤no pollution.The main contents of this research are as below:(1) Research on depositing process, investigating the effects of major parameters on coatings' characteristics.(2) Analyzing the morphologies, structures, composition distributions and phases of the coatings.(3) Investigation on the mechanic properties of TiN,CrN mono-layer films and TiN/CrN multi-layer film.(4) Study on the tribological behaviors and mechanisms of the hard coatings on AZ91D.(5) Study on the electrochemical corrosions and mechanisms of the hard coatings on AZ91D.After systemic experiments, the author concluded out the optimum depositing process parameters of TiN,CrN mono-layer films and TiN/CrN multi-layer film on AZ91D as thereinafter: The depositing temperature is 150～180℃, working time is 60～70min, distance between specimens and arc sources is 170～190mm, pressure is 0.5 Pa, the ratio of N₂: Ar is 10:1, bias voltage is 150～200V, arc current is 20～25A, duty cycle is 20%. A suitable depositing temperature is an important reference for the setting of other parameters, as the exact depositing temperature must below the temperature stability of 180℃of AZ91D. It was also found that geometry or size had a significant effect on the exact depositing temperature. Basing on the actual condition, the author corrected the measured temperature through calculation of heat-balance.Under the optimum process condition, the thickness of TiN,CrN and TiN/CrN films are 2.8μm, 3.6μm and 2.0μm respectively. All the films are dense, homogeneous, and with fine grains. But a few pin-holes and metal drop-lets were also spotted.The composition analyses indicated: between AZ91D matrix and TiN,CrN as well as TiN/CrN films, there were transiting layers with the composition of Ti,Cr and N changed in gradient. The thicknesses of those transiting layers reached several micrometers. The forming mechanism of the transiting layers is the diffusion behaviors of Ti,Cr and N with the reinforce of ion bombard. The existence of diffusion layer produced some effect of "pinning" and surely increased the adhesion strength between coatings and substrate.An adhere-peeling test had carried out according to ASTM D3359-78 to qualify the adhesion strength between coatings and substrate. No peeling off was observed, meaning the adhesion strength grade of three films all reached grade 5B. The results of scratching tests indicated: the adhesion strength of TiN,CrN and TiN/CrN films are 34N,32N and 43N respectively. Observing the morphologies of nicks, TiN film split severely, exhibited a more brittle behavior. TiN/CrN film exhibited a relative tough characteristic.The Knoop hardness of the films are as below: TiN film, HK_0.011433; CrN film, HK_0.011365; TiN/CrN multi-layer film, HK_0.011463. But analysis indicated that these hardness values are lower than the true values due to the deformation of substrates. A nanoindentation revealed that the hardness of TiN,CrN and TiN/CrN films are 17.2GPa,12.8GPa and 26.9Gpa, and the Young's modulus are 455GPa,399GPa and 423GPa respectively. The highest hardness of TiN/CrN film is the result of its finest grain size.A serious of ball-on-disc sliding experiments were done under the condition of room temperature and lubrication-free. The wear track of polished AZ91D exhibited adherence, scratching, tearing and features of plastic floating. The width of the track is near 2mm, and the major mechanism of wear is adherence. Whereas, the tracks of the specimens coated with TiN,CrN and TiN/CrN films are relative smooth. The mechanism of wear is abrading. The average friction coefficient of polished AZ91D is about 0.28, but those of the specimens coated with TiN,CrN and TiN/CrN films are reduced to 0.09,0.13 and 0.08 respectively. The excellent tribological behaviors of the specimens coated with hard films may be attributed to the self-produced soft layer. Comparing to bare AZ91D, the specimens coated with TiN,CrN and TiN/CrN films have reduced wear volumes by 26～75 times, indicating that the nitride films can effectively improve the wear resistance of the magnesium alloy.Electrochemical experiments in 3.5%NaCl solution indicated, the corrosion potentials of the specimens coated with TiN,CrN and TiN/CrN films increased 90V,136V and 206V than that of bare AZ91D, and the corrosion rates decreased 33.6%, 45.2%, 85.9% than that of bare AZ91D, respectively. A neutral salt spray investigation was also conducted. Just 4 hours later, the surface of polished AZ91D sample was eroded in large area; in contrast, the time that the surfaces of samples coated with TiN,CrN and TiN/CrN films appeared etch-pits were prolonged to 48h, 52h and 64h respectively. The mechanism is inter-boundary corrosion. For the samples coated with TiN,CrN and TiN/CrN corrosion displayed where defects are in the films. Since Cl^- diffused through those place to the interfaces of films/ substrate, and then a corrosive cell was built up. The time that the diffusion takes is depend on the thickness and density of the films as well as adhesion between films and substrate, and most importantly is directly associated with the quantity and size of through-out pin-holes. Obviously, sample coated with TiN/CrN has few pin-holes due to its structure with a dozen of interface, so it exhibited a more favorable anti-corrosion behavior.Through systemic research, we can give out the conclusions of this study as below: Arc-glow plasma penetrating is an effective and practical process to improve wear resistance and corrosion resistance of magnesium alloy AZ91D, and TiN/CrN multi-layer coating performs best.