The vacuum PVD coating has very low surface friction coefficient, the friction coefficient between steel and polished surface of metal material is generally about 0.9, but the friction coefficient between PVD coating layer and steel is 0.01~0.6, the friction coefficient of PVD coating material (AlCrN, AlTiN) that is commonly used in die-casting mold is generally 0.4~0.6. The lower friction coefficient help reduce the surface friction between the mold with PVD coating and the machined parts, so the surface quality of the part is better than parts that produced by the uncoated mold.
The production of die-casting molds needs the harshest conditions. In high-temperature of 600-800°C, after high-pressure injection and the non-stop thermal expansion of mold surface, the service life of the die-casting mold is very short, and it needs continuous repair and maintenance in the production process. The main causes of die casting failure are cracking, erosion, sticking materials, deformations and so on.
Since the mold cavity works at high temperatures, the improvement of die casting mold performance need the characteristics that within the service life of the mold, the accuracy and deformation of the cavity surface must be maintained under alternating conditions of high and low temperatures. Therefore, the material of die casting mould is not only has the characteristics of plastic molds, but also has advantages of excellent high temperature resistance, good hardness, great oxidation resistance, good temper resistance and impact toughness, great thermal conductivity and fatigue resistance. The traditional die casting mold adopts the quenching, tempering and polishing treatment process, and the hardness of the material itself is limited to be increased. At the same time, the working temperature is close to or exceeds the tempering temperature, and it causes the second tempering of the mold and then leads to hardness decrease and the mold deforms.
Vacuum PVD coating can solve some of the problems encountered in die casting molds by depositing a layer of coating on the surface of the mold, this type of coating is characterized by its high thickness and good temperature resistance. Vacuum PVD coating can effectively improve the high temperature resistance, surface hardness and oxidation resistance, at the same time, while the vacuum PVD coating on the surface can resist the impact of metal liquid. The usual die casting mold coatings include TiAlN, AlCrN, AlTiCrN and so on. And the common idea is using a layer of hard vacuum PVD coating to resist the high temperatures brought by the metal liquid and the flushing of the mold.
The above several kinds of coatings have better high temperature resistance and the coefficient of friction against steel below 0.5, which can effectively solve the deformation caused by sticking materials and quenching heat. Also, the hardness of these coatings is higher than HV3000, and the coatings can effectively maintain the hardness at high temperatures, they can also resist the stress and deformation caused by high-temperature metal liquid on the mold.