Effect of boronizing pretreatment on CVD diamond o

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Effect of boronizing pretreatment on smooth cemented carbide substrate on the properties of CVD diamond film

Abstract: improving the adhesion and finish of diamond film is the key factor to realize the wide application of CVD diamond coating in the field of tools and wear-resistant devices. Diamond film was deposited on the surface of smooth WC Co cemented carbide substrate by hot wire CVD. The effect of boronizing pretreatment on inhibiting co graphitization on the surface of smooth substrate and ensuring the adhesion of diamond coating was studied. The results show that the boronizing pretreatment method can not only avoid the serious damage to the smooth substrate surface caused by grinding, etching and chemical corrosion, but also effectively inhibit the adverse effect of CO on the diamond film, and obtain a smooth diamond film that meets the requirements of adhesion, which is of great significance for expanding the application field of diamond film

I. Introduction

cvd diamond film has very excellent performance, which is close to the hardness and wear resistance of natural diamond. The application of diamond film in cemented carbide tools shows its superior cutting performance, and the tool life has also been greatly improved. However, the adhesion between CVD diamond film and substrate and the smoothness of diamond film have always been an important reason to hinder the wide application of diamond film. To solve this problem, scientific and technological workers at home and abroad have carried out extensive research from the aspects of grinding, etching and CO removal, plasma etching, the formation of intermediate compounds and intermediate transition layers of CO, and developed a series of new pretreatment technologies. The use of these technologies can eliminate the adverse effects of co bonding phase, improve the nucleation density of diamond films, and improve the adhesion between films and substrates

however, the widely used pretreatment technology at present generally aims at coarsening and CO removal. While effectively improving the nucleation density of diamond films and the adhesion between the film and the substrate, the coarsened substrate morphology will be reflected on the surface of the deposited diamond films, which will directly affect the flatness and smoothness of the films. Therefore, the research and development of new pretreatment technology that can not only ensure the adhesion of diamond films, but also not damage the surface finish of the original substrate is one of the key problems to be solved in depositing ultra smooth diamond films that meet the practical requirements on the surface of precision and smooth cemented carbide

in this paper, a compound layer of CO is formed on the surface of the smooth cemented carbide substrate by boronizing pretreatment. This layer can not only ensure the smoothness of the substrate surface, but also passivate the CO in the substrate, so that the purpose of obtaining smooth diamond film can be achieved on the premise of ensuring the adhesion of diamond film and the smoothness of the substrate surface

II. Test method

YG6 smooth surface cemented carbide blade is used as the substrate material in this test. The surface of the substrate material is polished, and the surface roughness ra=0.034 M. Boronizing pretreatment adopts solid boronizing method, boronizing adopts external heating high-temperature orange exhaust, heating adopts iron chromium aluminum furnace, the crucible is made of 4 ~ 6mm 1Cr18Ni9Ti corrosion-resistant stainless steel plate, and double-layer cover plate is used to ensure its tightness. Different boronizing agents were used in boronizing. In order to prevent damage to the surface finish of the sample, diamond film is deposited without any roughening and grinding after boronizing pretreatment. See Table 1 for test parameters. Table 1 boronizing pretreatment test parameters

sample code substrate material boronizing time

(H) boronizing temperature

(℃) treatment method sample 1W (WC) =94%

w (CO) =6% (non boronizing) (non boronizing) untreated sample 2 pickling for 60min, W1+w5+w10

diamond powder grinding 20min sample 36950w (B4C) =71%

w (kbf4) =14%

w (Na2CO3) =15% sample 4W (B4C) =71%

w (kbf4) =14%

w (rare earth) =5%

w (Na2CO3) =10%

in this paper, diamond film is deposited by hot wire CVD method, and the reaction gas for CVD diamond film test is acetone.The volume fraction of hydrogen and acetone is 1%, the reaction pressure is 5.32kpa, the hot filament is tantalum, and the surface temperature is about 2200 ℃, The bias current is 0.1 ~ 0.3A, and the substrate surface temperature is 750 ~ 900 ℃. The two filaments are arranged in parallel with the substrate, the filament spacing is 22mm, and the substrate is in the middle. The deposition time is 4 ~ 5h. The boronizing effect of the sample, the microstructure after boronizing and the surface morphology of the diamond film were observed and analyzed by s530sem scanning electron microscope and D8 discover gadds X-ray diffractometer. The adhesion of the diamond film was evaluated by surface Rockwell hardness tester (120 ° conical diamond indenter, load 150 ~ 1500N). Through the above methods, this test is comprehensively analyzed and evaluated

II. Results and analysis

· surface morphology and composition analysis after boronizing pretreatment

the substrate material is smooth YG6 cemented carbide blade. Figure 1A shows the surface morphology of the original YG6 cemented carbide, with smooth surface and no obvious depression and bulge; The acid pickling and roughening treatment of the substrate is to corrode the substrate surface with dilute hydrochloric acid for 60min to remove Co. after roughening, grind it with 10mim Al2O3 sandpaper, mix it with diamond powder with particle size of W1, W5 and W10 in the ratio of 1:1:1, grind the substrate surface for about 20min, and then use acetone bath for ultrasonic cleaning. The purpose is to improve the nucleation density of diamond and increase the adhesion of diamond film. Fig. 1b shows the surface morphology of sample 2 after acid pickling and roughening, with uniform micro bumps on the surface and obvious exposure of WC particles; Sample 3 and sample 4 were boronized. Solid boronizing method was used for boronizing pretreatment. Figure 1c and figure 1D are the surface morphology of sample 3 and sample 4 respectively. It can be seen that sample 3 has dark substrate color and holes due to deep boronizing; The surface holes of sample 4 after boronizing are less, the particles are fine, and the boronizing layer is continuous

(a) (b)

(c) (d)

Fig. 1 SEM of substrate surface after different pretreatment

the measurement method spectrum of XRD wear parameters of substrate surface after boronizing pretreatment is shown in Fig. 2. Fig. 2a is the XRD diagram of sample 1, that is, the original smooth cemented carbide. Fig. 2B and Fig. 2c are the XRD diagrams of sample 3 and sample 4 after boronizing, respectively. It is observed from Fig. 2B and Fig. 2C that the wave peak of w2cob2 appears after boronizing pretreatment. This research achievement will contribute to the research and development of shape memory polymers and liquid crystal elastomers, and the wave peak of WC will be weakened. In the process of boronizing, boron carbide (B4C) is the boron donor, and potassium fluoborate (kbf4) and sodium carbonate (Na2CO3) are the activators

Figure 2 XRD diagram of substrate surface after boronizing pretreatment

when boronizing begins to heat, the boron donor and activator react to form active B atoms. The reaction process is as follows:

kbf4=bf3+kf (1)

2bf3+2bc4+o2=3bf2 ↑ +7[b]+2co (2)

2kf+b4c+o2=k2o+bf2 ↑ +3[b]+co (3)

BF2 is an unstable gas, which has been analyzed.Boron atoms are produced to form stable BF3 gas, The reaction formula is

3bf2=[b]+2bf3 ↑ (4)

the BF3 obtained in formula (4) can react with B4C to produce active [b] atoms, so it goes round and round, and finally the boron donor B4C is consumed. The active boron atoms generated by the reaction will be adsorbed to the surface of cemented carbide and react with CO and WC to form compounds such as w2cob2. The reaction formula is

2w+co+[b]=w2cob2 (5)

due to the continuous production of [b] atoms, it forms a stable compound with the co diffused from the inside of the substrate to the surface, thus eliminating the adverse effects of CO in the deposition process of diamond, and the formed boronizing layer can effectively prevent the internal co from diffusing to the surface. At the same time, WC, as the main body of cemented carbide, also exists in the boronizing layer, which ensures the continuity of the substrate, There is no other problem of applying adhesion between the transition layer and the substrate. The surface composition of sample 2 basically did not change because it was only roughened by pickling and grinding; The mass fractions of WC and w2cob2 in sample 3 are 68.5% and 31.5% respectively; The mass fractions of WC and w2cob2 in sample 4 are 65% and 35% respectively. The main reason why the surface boron content of sample 4 is higher than that of sample 3 is the addition of rare earth, which has an obvious catalytic effect in the boronizing process, that is, more co forms compounds on the surface of the substrate, thus forming a dense boronizing layer

· surface morphology and adhesion of diamond film

the SEM diagram of the surface morphology of CVD diamond film is shown in Figure 3. Figure 3a is the surface morphology of the diamond film deposited by sample 1. Without any pretreatment, the diamond film cannot be effectively nucleated during the deposition process, so the film formation is poor, and the adhesion is very poor, and the phenomenon of falling off occurs at the edge. Fig. 3b is sample 2. After pickling and roughening, the diamond film has coarse and continuous grains, clear crystal shape, well-developed crystal shape, showing a typical (111) crystal surface, and the surface is relatively rough; Figure 3C shows the surface morphology of the diamond film deposited after boronizing pretreatment of sample 3, showing a (100) oriented diamond film with slightly poor crystallinity; Sample 4 was pretreated by Boriding with rare earth. The diamond particles were spherical crystal, with uniform and continuous surface, high nucleation density and small surface roughness. It can be seen from Figure 3 that the content of w2cob2 formed on the surface of sample 4 is high due to the catalytic effect of rare earth during boronizing, which effectively blocks the seepage of CO and forms a fine-grained diamond film

Fig. 3 SEM image of CVD diamond film surface after deposition

in this paper, the adhesion of the deposited diamond film is evaluated by Rockwell indentation method, and Fig. 4 is the indentation photo under the same load of 1000N. It can be seen from the figure that sample 1 in Figure 4A has no pretreatment, so the adhesion of diamond film is very poor, and the suitable viscosity should be selected first when large pieces of film fall off; Sample 2 in Fig. 4B has been roughened by pickling, and there is no obvious divergent crack around the indentation, but the diameter of the film falling off area around the indentation is large; In Figure 4C, there are some divergent cracks around the indentation of sample 3, and the surrounding film is partially broken, with poor adhesion; In Figure 4D, the film falling off area around the indentation of sample 4 is significantly smaller than that around the indentation of sample 2, with few divergent cracks, and its adhesion is higher than that of sample 2; The indentation area of sample 4 is smaller than that of sample 3, and no obvious divergent cracks are found. Therefore, it can be determined that sample 4 has the strongest adhesion. The reason is mainly due to the addition of rare earth in sample 4, which has the effect of catalytic infiltration. The boronizing of the substrate is deep, which alleviates part of the surface stress and enhances the adhesion, which is conducive to the deposition of diamond films and the improvement of adhesion. It can be concluded that the adhesion of sample 4 with boriding pretreatment method and rare earth added in boriding agent is better than that of sample 2 after acid pickling and grinding. On the premise of ensuring the adhesion, diamond thin film with smooth surface can be obtained by boriding pretreatment

Figure 4 SEM image of diamond film indentation (load 1000N)

4 Conclusion

in this paper, a smooth diamond film is prepared on the surface of smooth cemented carbide by hot wire CVD with boronizing pretreatment method. The new boronizing pretreatment method not only does not damage the accuracy and smoothness of the smooth surface of the original substrate, but also ensures the adhesion of the substrate, which is conducive to improving the smoothness of the diamond film according to the plan

when the boronizing temperature is 950 ℃, the boronizing time is 6h, the boronizing agent is w (B4C) =71%, w (kbf4) =14%, w (rare earth) =5%, w (Na2CO3) =10%, the adhesion effect of the diamond coating deposited after pretreatment on the smooth cemented carbide surface is the most ideal, and its adhesion is better than that after pickling and grinding. At the same time, rare earth plays an catalytic role in the boronizing process, increasing the boride content on the substrate surface, Beneficial to diamond film

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