Mechanical properties and wear resistance of multi-layer coatings for cutting tools

During the cutting process, the cutting force of the tool is 2~3GPa, the cutting temperature is up to 900~1 100 °C, and the cutting speed is usually in the order of tens of meters to several hundred meters per minute, so under high pressure, high temperature and high speed The problem of friction and wear of working cutting tools is very serious. Hard coatings play an important role in improving cutting performance and extending tool life. The most studied to date is the TiN coating, which has high hardness, low friction and good chemical stability. Ti(C, N) coatings have better anti-adhesion and thermal wear resistance than TiN coatings. In addition to having a low coefficient of friction, the wear-resistant coating must have high microhardness, high toughness and adhesion to the substrate. By introducing a fixed number of intermediate transition layers parallel to the substrate, the toughness and hardness of the coated tool can be improved to prevent crack initiation. Studies on TiN multilayer coatings have shown that it has better tribological properties than a single coating. Su et al.'s studies on the wear resistance and cutting performance of multilayer TiN/Ti(C, N) coated tools show that they perform better than single layer coatings. The wear resistance and reliability of the coating are often governed by its mechanical properties. Due to the interaction between the membrane, the interface and the matrix, it is difficult to assess the mechanical properties of the coating. The advent of nanohardness testers allows people to understand the mechanical properties of coatings from the microscale (nanoscale) history. The authors used a nano hardness tester to analyze the deformation, failure and wear resistance of the four coatings.

1 Test method

Test device

The test device was produced by CSEM Instruments, Switzerland. The system consisted of a nanohardness tester (NHT) and an atomic force microscope (AFM), and each optical microscope accessory was installed. Components such as the indenter and the optical microscope for selecting the sample and observing the indentation are controlled by an electromechanical positioning system with a displacement resolution of μm in the vertical direction. A load is applied to the pressure bar by an electromagnetic force generated by an electromagnetic coil mounted on a pressure bar supported by a guide spring, which is a standard Vickers diamond indenter. The displacement of the plunger is measured with a capacitive sensor. The load and indentation depth resolution of the entire system were 10 μN and 1 nm, respectively. During loading and unloading, precise positioning of the indenter and the sample surface is achieved by a sapphire ring that is always in contact with the surface of the sample to be tested.

Test sample

Preparation of TiN, TiN/Ti(C, N)/TiC, TiN/Ti(C, N)/TiC/Ti(C, N)/TiC and TiN/Ti(C, N on cemented carbide substrates by CVD technique ) / TiC / Ti (C, N) / TiC / Ti (C, N) / TiC and other four wear-resistant coatings. The raw material of the cemented carbide substrate is passivated, washed, furnaced and heated with 99.50% H ₂ , 99.99% N ₂ , 99.99% CH ₄ , 99.50% CO ₂ , chemically pure TiCl ₄ and AlCl _{3 ,} etc. The CVD coating is applied and cooled to produce a sample of the coating to be tested. The thickness of the four coatings is 4.0 μm, 1.5 μm / 1.0 μm / 1.5 μm, 1.5 μm / 1.0 μm / 1.5 μm / 1.0 μm / 1.5 μm and 1.5 μm / 1.0 μm / 1.0 μm / 1.0 μm / 1.5 μm / 1.0 μm / 1.5 μm.

2 Test results and discussion

Mechanical properties

The indentation test was performed on the four coatings using a nano-hardness tester to obtain the relationship between the load and the indentation depth during loading and unloading. E is the modulus of elasticity, and HV is the Vickers hardness value of the coating, which is determined according to the method of Oliver et al. In addition to considering the unloading curve, the method also considers the shape of the indenter and the depth of the indentation to calculate the contact area under load. The hardness is regarded as the average pressure of the material during the unloading process. Multilayer coatings have better load carrying capacity than single layer coatings. Li et al. used a nanohardness tester to analyze various cracking processes occurring on the surface of the coating during the indentation process. It was found that the high stress in the contact zone caused the first approximately annular crack of the penetrating film layer around the indenter; the high side The pressure causes the coating/substrate interface to peel and break at the contact zone; at the edge of the bent film, a second approximately annular crack or cracked piece of the penetrating film layer occurs due to the bending stress. In the first stage, if the coating exhibits a crack in the approximately annular penetrating film layer, a corresponding step will appear on the ph curve, and vice versa. We studied the failure characteristics of the four coatings. It can be seen that as the indentation load increases, a step appears on the ph curve, showing that cracks in the coating that are approximately annular through the film layer are initiated. Each step corresponds to a coating of an approximately circular crack penetration film layer, thus defining p _f at the step of loading the coating Failure critical load. Thus, the fracture failure critical load of the four coatings obtained by the indentation curve is 11.1 mN, 16.4 mN, 35.5 mN and 56.3 mN, respectively. It can be seen that the fracture failure load of the multilayer coating is significantly higher than that of the single-layer TiN coating; as the number of coating layers increases, the critical load psub>f value increases. This is because the intermediate layer in the multilayer coating prevents the initiation and propagation of cracks (the ability of the intermediate layer to prevent crack initiation and expansion is related to its thickness and number of layers).

According to a document, the fracture toughness Ksub>IC of a coating can be calculated by the following formula:


E and v are the elastic modulus and Poisson's ratio of the coating; 2pR _C is the length of the crack in the coating; t is the thickness of the coating; U is the change of the strain energy before and after the occurrence of the crack. The area on the ph curve reflects the elastoplastic deformation energy of the coating/matrix system, and the strain energy U released when the first crack of the annular penetrating film layer is generated can be calculated from the product at the step on the curve. Kazmanli et al. also describe the relationship between the steps on the ph curve and crack formation. It is calculated by the formula (1) may be obtained four kinds of coating fracture toughness of ^{1.51MPa · m ½, 2.18MPa · m} ½, 3.4MPa · m ½ and 3.9MPa · m ^½. It can be seen that as the number of coating layers increases, the fracture toughness value increases. However, the use of multi-layer coatings increases the complexity and cost of the process, so the appropriate number of layers should be chosen. For this we recommend the use of TiN/Ti(C, N)/TiC/Ti(C, N)/TiC coatings.

The ph curve describes the failure of the coating fracture; and the ph ² curve can reflect the change of the coating/substrate boundary before the fracture of the anti-friction wear-resistant coating, especially the interface change between the multilayer coatings. For the monomer phase material, the plastic deformation component in the indentation depth is h _p and the elastic deformation component is h _e , then the total indentation depth h:

f and y are parameters related to the geometry of the indenter; p is the load; HV is the hardness; and E is the modulus of elasticity.

Therefore, p=Kh ² can be obtained, and K is the elastoplastic parameter of Loubet. The indentation process for a single bulk material, p∝h ² . When studying the coating/matrix system, it was found that the straight line segment from the origin to the inflection point on the typical ph ² relationship curve conformed to the p∝h ² relationship, reflecting the elastoplastic deformation of the coating. According to the analysis of Hertz contact theory, it is found that the maximum shear stress is still in the pressed coating, and the matrix can yield yield, so the straight line segment reflects only the deformation of the coating. After crossing the inflection point, the high shear stress causes the matrix to yield, so that the coating is bent, the interface changes, some interfaces are desorbed during the unloading process, and material accumulation occurs around the contact zone under tensile stress until Cracks appear at the steps. Therefore, the critical load of the coating boundary is represented by the load p _i at the inflection point. The ph ² and ph curves completely reflect the entire process of coating interface change and fracture failure. The ph ² curve of the four coatings, the dashed line is the line conforming to p∝h ² , the solid line is the ph ² curve during the pressing process, and the inflection point is located at the separation point between the solid line and the broken line. The line segment from the origin to the inflection point reflects the deformation of the coating itself, and the load value at the inflection point is lower than the load value at the step. It was found by SFM observation that cracks appeared on the surface of the coating under the corresponding step load. It can be seen from the indentation test data that the interface change of the single-layer TiN coating at p _i =3.13 mN indicates that the interface bonding of the single-layer coating is weak and the toughness of the coating is also poor. The TiN/Ti(C, N)/TiC coating showed an interfacial change at p _i = 7.5 mN. From the origin to the step are straight segments (real and dashed coincident), indicating that the two coatings have obvious interface changes before and after the failure. Therefore, TiN/Ti(C, N)/TiC/Ti(C, N)/TiC and TiN/Ti(C, N)/TiC/Ti(C, N)/TiC/Ti(C, N)/TiC The layer coating has high interfacial strength and good toughness.

Wear resistance

The surface of the brittle coating material breaks, peels and breaks during the friction process. At this time, the wear resistance of the coating mainly depends on the material's ability to resist brittle fracture. Therefore, increasing the strength and fracture toughness of the material can improve its wear resistance. Considering the quality factor of the material (the temperature and chemical wear of the friction zone are not considered here, if the temperature influence is required to be corrected), the wear resistance W _{R of the} coating material can be expressed as:

Where: W _R is wear resistance; K _IC is fracture toughness (MPa·m ^1⁄2 ); E is elastic modulus (GPa); HV is hardness (GPa). The table below lists the wear resistance of the four coatings calculated according to formula (4). It can be seen that TiN/Ti(C, N)/TiC/Ti(C, N)/TiC/Ti(C, N)/TiC coatings have the best wear resistance, and the results are consistent with the results of the cutting test. . The cutting test results show that the TiN/Ti(C, N)/TiC/Ti(C, N)/TiC/Ti(C, N)/TiC coated tools have the longest service life among the four coatings examined.
3 Conclusion

The formation of cracks in the coating has a good correspondence with the load and the step on the indentation depth curve.

The mechanical properties of the coating material can be fully described by the flat force curve and the load and indentation depth curves of the load and the indentation depth. The steps on the load and the indentation depth curve can be used to describe the fracture failure of the coating, and the sinking line on the square of the load and the indentation depth can be used to describe the interface variation of the multilayer coating. The fracture failure and interface change of the coating can be described by the critical loads p _f and p _i , respectively.

Multilayer coatings have high hardness, fracture toughness and wear resistance. As the number of coating layers increases, the ultimate load p _f and p _i values ​​tend to increase. Among them, TiN/Ti(C, N)/TiC/Ti (C, N)/TiC/Ti(C, N)/TiC coatings have the best mechanical properties and wear resistance.

Led Mirror Light

Led Illuminated Mirror Light,Over Mirror Light,Decoraport Led Mirror,Rgb Led Strip

NINGBO EASTKEY ILLUMINATE APPLIANCE CO.,LTD , https://www.eastkeylighting.com