4 Results and discussion
(1) The necessity of pretreatment of the substrate surface before coating
The main component of the η phase is W 3 CO 3 C. When a continuous Co film is present on the surface of the substrate, Co is very easily oxidized as long as there is a little H 2 O present. See the chemical reaction formula below.
Co+H 2 O→CoO+H 2 ↑
CoO+WC→W 3 CO 3 C(η phase)+CO↑
It can be seen from the above equation that it is necessary to have Co, WC, and H 2 O simultaneously to form the η phase (W 3 CO 3 C). It is possible that some of the WC will dissolve in the Co phase, but it is impossible to dissolve Co in the WC. Thus, on the WC particles, since no Co exists, it is impossible to form the η phase. Therefore, as long as a small amount of H 2 O is present in the Co phase (which is also impossible to completely avoid), an η phase is formed on the surface. That is, if a continuous η phase is not desired, a continuous Co film should be avoided on the surface of the substrate. The continuous Co film must be removed before the coating.
(2) Select TiCN as the transition layer
In fact, TiC has a thermal expansion coefficient closer to that of the matrix than TiCN compared to the cemented carbide substrate. But we can see that a serious η phase is formed between TiC and the matrix. Because when Ti and CN 4 enter the reaction chamber at the same time, Ti not only reacts with C in CH 4 , but also captures C in the alloy matrix, which causes the matrix to de-C, forming the de-carbon phase η phase, but if it is TiCN Direct deposition on the surface of the substrate is not the same. When Ti, N 2 and CH 4 enter the reaction chamber at the same time, since the reaction rate of Ti+N is much faster than Ti+C at high temperature, the TiN film will be formed first before the formation of TiC, and the formed TiN film will be wrapped on the surface of the alloy. Prevent Ti from taking C to the substrate, so that the substrate does not lose C, and the decarburized phase η phase is not formed.
(3) Al 2 O 3 thickness
There are chemical reactions at high temperatures:
Al 2 O 3 +H 2 O→Al 2 O 3 +HCl
This reaction rate is very fast. If the reaction is too fast, some of the formed Al2O3 does not have enough time to be ordered and crystallized on the surface of the substrate, but nucleates in the gas phase. Once the gas core is formed, most of the newly formed Al 2 O 3 will not be deposited on the surface of the blade but will be formed on the gas core. Because the gas core is small, but the number is huge, the total surface area is much larger than the surface area of ​​the insert, so only a small portion of Al 2 O 3 will deposit on the surface of the insert to form an Al 2 O 3 coating, while most Al 2 O 3 forms an Al 2 O 3 brown powder in the atmosphere due to gas phase nucleation. If we can prevent the formation of Al 2 O 3 gas nucleus and let all the generated Al2O3 deposit on the surface of the blade, we can get a thicker Al 2 O 3 coating at the same time. The catalyst is added just for Al. The formation of Al 2 O 3 gas nucleus is suppressed during the 2 O 3 coating process.
(4) Microstructure
It can be seen that the Al 2 O 3 coating is a granular structure, while the Al 2 O 3 coating is a complete columnar structure. This is because a large amount of gas phase nucleation products exist in the atmosphere during the formation of the granular structure, and they accumulate on the formed Al 2 O 3 coating, interrupting the growth of the crystal column and forming a granular structure. From the columnar Al 2 O 3 coating, since the gas nucleation is suppressed, the Al 2 O 3 crystal column can grow freely, and a complete columnar structure is formed. In addition, we can also see that there are many voids in the columnar structure, which prove that the gas phase nucleation is suppressed during the deposition of Al 2 O 3 , because once the gas core is present, those small particles will fill all the voids.
The addition of the catalyst is also very important. If the catalyst enters at the same time as the feedstock and maintains a constant amount throughout the deposition time, we will obtain an incomplete columnar structure.
(5) Adhesion fastness test
Due to the difference in the structure of the structure, we can see that there are different results in the test of adhesion fastness. The first group is easily loosened under the load of 4kg due to its granular structure determining the looseness of its Al 2 O 3 coating. Scratched. However, the second group of Al 2 O 3 coatings have an overall granular structure and are not interfered by gas nucleation during the crystallization of Al 2 O 3 . The structure is very solid and the surface can withstand a load of 7 kg.
5 Conclusion
The most important thing in the Al 2 O 3 coating is to prevent the formation of gas cores. A specific catalyst should be chosen to control the reaction rate of Al 2 O 3 . The amount of catalyst added is very important. If too much, the slow inhibition of the Al 2 O 3 reaction rate will affect the deposition efficiency of the Al 2 O 3 coating, but too little can not completely inhibit its gas phase nucleation. The key is to master the proportion of the catalyst. In addition, the way in which the catalyst is added during the Al 2 O 3 coating process is also important because the atmosphere in the reaction chamber is constantly changing, and the amount of catalyst required at different stages is different. effect.
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