Mon Jul 04 11:44:31 CST 2022
Today we are going to introduce several common wear and destruction ways of PCBN cutting tool
In cutting, the high speed relative motion between the cutting tool and the workpiece causes intense friction, and the hard point in the workpiece material has scratching effect on the tool surface. The tool wear caused by mechanical friction is one of the most common wear factors. Because the hardness of PCBN tool is much higher than the material being machined, its mechanical wear is not obvious.
The PCBN cutting tool is sintered by mixing CBN grains with binder. When cutting, the ceramic or metal as the binder is worn first, so that the CBN grain protrudes out of the tool surface and loosens the force until peeling.
Under certain conditions, CBN can react with oxygen, oxygen displaces nitrogen in CBN and generates B2O3, oxidation results in CBN crystal surface depression, crystal edge shrinkage, so that the tool "passivation" phenomenon. CBN began oxidation at 650℃ (at which N2 was released), and oxidation intensified at 1035℃. The chemical reaction formula was 4BN(CBN)+3O2→2N2↑+ 2B2O3
Water can also react with CBN at high temperature, and its chemical reaction formula is BN(CBN)+3H2O→H3BO3+ NH3
Because this "hydrolysis" can lead to CBN wear, water cutting fluids should generally be avoided when using PCBN tools.
The reverse transformation of CBN→HBN(hexagonal boron nitride) will occur at 1234℃. This transformation starts from the grain boundary microcrystalline region, and the part that has been converted to HBN loses its cutting ability due to the very low hardness, which is easy to be taken away by the "hot chip flow" of high-speed motion, resulting in tool wear of PCBN. This wear is called reverse transformation wear (also known as phase change wear). PCBN tool in high temperature (>1200℃) after cutting for a period of time, the tool part of the high temperature area will sometimes appear by many small pits composed of uneven "pitting", this is because the cutting temperature exceeds the critical temperature of CBN to HBN transformation. The white "pitting spots" on the tool surface after the reverse transformation wear are actually the transformed HBN remaining after the single crystal of CBN falls off. In the inverse transformation from CBN to HBN, oxygen and oxide act as catalysts. On the contrary, cobalt inhibits the CBN to HBN transition by decreasing the oxidation atmosphere.
PCBN tool in high temperature, high pressure, high speed cutting conditions, tool working layer and processed material and the surrounding medium chemical reaction, when the reaction product is dissolved, on the tool surface will form a layer of liquid film, Its components are mainly oxides, carbides, nitrides, borides generated by chemical reactions (such as B2O3, Fe-FeB2 eutectic), and some intermetallic compounds. This liquid film has great influence on the wear of PCBN tool. When the cutting speed is low, the viscosity of liquid film is large, easy to be taken away by chip bonding, so the tool wear is more serious; As the cutting speed increases, the cutting temperature rises, the dynamic viscosity of the liquid film decreases, the friction between the insert and chip can play a significant lubrication role, and BN in the film has been saturated, at this time the liquid film can play a protective layer, to prevent the further development of component diffusion and chemical wear, so the tool wear is small. The cutting experiments show that the higher the Al content in the tool binder is, the faster the wear rate of the tool surface is and the shorter the tool life is.
CBN is chemically inert to iron group elements (Fe, Ni, Co, etc.). Some studies have shown that there is no mutual diffusion between CBN grains and electrolytic iron in the diffusion experiment (1200℃, heated for 30min). In the diffusion experiment between PCBN and 55 steel (1200℃, 30min), it was found that the B and Co diffused slightly into Fe after CBN crystallized. In addition, the heating experiments show that Al diffuses with Ni in the material processed by TiN and TiC PCBN tools. The Co in the Co based PCBN tool and The Ni in the machined material also diffused mutually. If the tool material contains Ni, the diffusion wear is more serious. In addition, when the PCBN tool binder contains Al and the processed material contains Si, Si will diffuse into the tool and combine with Al to form SiAlON, resulting in tool wear. Studies have shown that the mutual diffusion strength between several tool materials and iron is in descending order: diamond → silicon carbide → cubic boron nitride → alumina; The order of their mutual diffusion strength with titanium alloy is just opposite, respectively: silicon oxide → cubic boron nitride → silicon carbide → diamond.
Although CBN is chemically inert to iron group elements, this is not the case for other elements. PCBN cutting tool under certain pressure and temperature conditions, with the flow, chip point and processed materials are constantly exposed surface of fresh, inevitably produce mutual diffusion between elements, the spread of CBN inert decrease as a result, and alloy elements increasing tendency of affinity, and create conditions for adhesion wear. Due to the cutting chip, workpiece and tool before, after the surface of the fierce friction and pressure, prompting them to bond. When the relative movement of the two sides causes the material in the bond zone to fracture and be taken away by one side, it causes the bond wear of the PCBN tool. The results show that bond wear usually occurs in the form of particle shedding. Metal Ni will increase the bond strength between the tool and the workpiece material, thus aggravating the bond wear.
PCBN is composed of numerous small and unoriented single crystals of CBN. In the process of CBN polycrystallization, some "impurities" (such as Si, Ca, Cu and other elements) are diffused into the material through catalysts or additives, and these "impurities" exist at grain boundaries. Because the grain boundary is the impurity rich area, the strength is relatively weak, in a sense can be regarded as "crack" (called "fine crack"). In addition, there are internal stresses in the original grains and at grain boundaries under either congenital or processing conditions (even if sintered well). The "fine crack" and internal stress cause the actual strength of polycrystalline to be much lower than its theoretical value. When the PCBN tool is cutting, the peeling off of small single crystal particles on the blade is called microcracking, and the peeling off of several CBN particles is called microcracking edge. The mixed wear of micro-cracking and micro-cracking is a special wear type of superhard tool materials.
When cutting PCBN tool, due to the friction and scraping of hot chip flow, the micro-impact caused by uneven material processing, the vibration of machine tool - workpiece - tool system and other factors, the polycrystal first crack at the grain boundary, the discontinuous shedding of single crystal particles causes micro-cracking and micro-collapsing edge of the tool. A convex and concave cracking zone is formed at the cutting edge and expands continuously until the fracture is caused.
Abnormal wear mainly refers to the PCBN tool edge breakage breakage (CBN mass caving). The cause of breakage is related to improper selection of cutting conditions, unreasonable use of cutting tools, poor processing equipment conditions, operator's lack of experience and other factors (sometimes also related to the quality of composite pieces). The low quality of cutting tool grinding is also an important factor causing tool breakage. The scratches left on the surface of the tool during grinding will greatly reduce the strength of the tool, and then will make the CBN grain fall off from the scratch, resulting in micro-cracking wear and micro-chipping edge of the tool, until the tool is damaged.