Experiment on wear-resistant particle wear properties of ceramic phase arc sprayed coatings

       Experimental part



Preparation of coating samples



The coating was prepared by JZY2250 arc spraying equipment. The spraying material was a self-made Fe-based powder core wire with a diameter of 2 mm. The outer skin of the wire was a 10.0 mm×0.3 mm 304L stainless steel strip. It is (mass fraction, the same as the full text): 0. 027%C, 0. 36% S, i 1. 65% Mn, 0. 027% P, 0. 001% S, 18. 18% Cr, 8. 06% The balance of N and i is Fe. The main components of the wire powder core are shown in Table 1. The particle size of WC212CoNi composite powder in the powder core is 0.038~0.075 mm, and the particle size of Fe62 iron-based self-fluxing alloy powder is 0.045~ 0. 11 mm, its chemical composition is: 4. 5% C, 40% Cr, 1. 5% S, i 1. 8% B, the balance of Fe (all products of Beijing General Research Institute of Mining and Metallurgy). Base material It is Q235 steel, the size is 57 mm × 25 mm × 5 mm, and the surface is sandblasted and sprayed with a coating thickness of 1.5 mm. The arc spraying process parameters are: arc voltage 28~32 V, working current 160~200 A , The compressed air pressure is 0.5~0.6MPa, and the spraying distance is 100~200 mm.



Coating performance test



TH320 type full Rockwell hardness tester was used to measure the Rockwell hardness of the surface of the sample. The sample was ground and polished, and the coating thickness was about 1 mm. According to the provisions of GB8640288, an N scale (diamond cone indenter) was used, and the total load was 441.3N. (HR45N), loading time 5 s, dwell time 3 s, 5 points are continuously measured for each sample, and the arithmetic mean is taken. The distance between two indentation centers or the distance between any indentation center and the edge of the sample is not less than 3 mm. The composition and phase of the coating were analyzed by D8 ADVANCE X-ray diffractometer (XRD) produced by BRVKER/AXS, Germany. The diffraction conditions were CuKα target, 40 kV and 20 mA. The size of the sample was 10 mm×10 mm , the coating thickness is 1 mm. The abrasive wear test uses the MLS2225 wet rubber wheel abrasive wear tester, which is a typical three-body abrasive wear, and the abrasive (quartz sand) flows on the surface of the rubber wheel and the specimen to cause wear. The test parameters are as follows: the speed of the rubber wheel is 240 r/min, the hardness of the rubber wheel is 60 (Shore hardness), the load is 100 N, the abrasive is quartz sand of 0.212-0.425 mm, the pre-grinding is 1 000 r, and the wear time is 250 s. , fine grinding 2 000 r, wear time 500 s. Before and after the test, put the test piece into a beaker filled with acetone solution, clean it in an ultrasonic cleaner for 3-5 min, and dry it with a stopper with an accuracy of 0.1 mg. Doris BS224S electronic balance weighs the mass loss before and after wear, and takes the average value of 3 samples to measure the wear resistance of the material. At the same time, Q235 steel is used as a comparison to compare the wear mass loss of the comparison piece and the wear of the measurement piece The ratio of mass loss was taken as the relative wear resistance of the formulation. The coating surface and its wear scar surface morphology were observed by optical microscope and scanning electron microscope (SEM).



  Results and discussion



The structure and properties of the coating



The photo of the microstructure of the coating cross section. It can be seen that the coating has a typical layered structure. The microstructure of the 1#, 2# and 4# coatings is roughly the same as that of the 3# coating. There are many fine pores, 2# and 3# coatings are relatively dense, and there are fewer pores in the coatings, while the 4# coating has more pores, and the degree of densification decreases. The porosity of 1#~4# coatings 3. 63%, 2. 72%, 1. 63% and 3. 22%, respectively. Figure 2 shows the XRD pattern of the 3# coating. It can be seen that the coating is mainly composed of (Fe, Ni), Ni2Cr2Fe, Fe3B and WC and other phases, and also contains a small amount of Fe3O4, which is due to the high temperature of the arc zone and the oxidation of the material during the arc spraying process. The XRD patterns of the 1#, 2# and 4# coatings are similar to those of the 3# coating. same.



Surface Rockwell hardness and abrasive wear test results of the coatings. It can be seen that the hardness of the four coatings increases gradually with the increase of the WC content in the coatings. When the WC content does not exceed 25%, the wear resistance of the coatings increases. It increases with the increase of WC content. When the WC content exceeds 25%, the wear resistance of the coating decreases. The wear resistance of the coating mainly depends on the compactness and phase structure of the coating. As the WC content increases, the FeCrNiB matrix decreases. A large number of massive WC particles appear in the coating. When the amount of WC is more than 25%, the pores in the coating increase, the anchorage bonding performance between the hard phase and the matrix decreases, and micro-cracks appear in the direction perpendicular to the grinding direction, which is not conducive to the wear resistance of the coating. Sexual enhancement.



The SEM photo of the coating structure and its local energy spectrum analysis results. Combined with the XRD and EDS analysis results, it can be seen that the white local area in the SEM photo [point A in Figure 3(a)] is the residual WC hard phase inside the coating , its composition is: 4. 68% C, 5.16% O, 1. 57% Cr, 0. 77% N, i 13. 64% Fe, 74. 18% W. Black area [B in Fig. 3(a) 1. 98% C, 3. 75% O, 10. 69% Cr, 4. 23% N, i 63.21% Fe, 6. 16% W, 9. 98 %B.



Wear surface analysis



Shown are the SEM photos of 1# and 2# coatings and the worn surface morphology of Q235 steel. The worn surface morphology of 3# and 4# coatings is basically the same as that of 2#. It can be seen that Q235 steel and 1# coating are in Under the micro-cutting of hard abrasive particles, furrows of different depths and widths are formed in the direction of abrasive particle movement, showing typical plastic cutting characteristics. Cutting and peeling off, so the wear surface of the coating is relatively smooth. Since the 1# coating has less WC added to the wire, only a small amount of WC hard phase remains in the coating, which cannot prevent the quartz sand abrasive grains from cutting the matrix. When the abrasive grains cut the spray coating at an acute angle, the metal on both sides of the cutting groove will have a large plastic deformation. Under the repeated action of other abrasive grains, the wrinkled and uplifted metal on both sides of the furrow will easily form fragments and peel off, forming large and small pieces in the spray coating. Unequal spalling pits. Spalling mainly occurs in the area with serious wear scars. At the same time, the matrix produces large plastic deformation under the action of large-sized hard abrasive particles, which is conducive to the formation of high-density dislocations in the surface layer. Under the repeated action of grains, dislocations accumulate to form voids, which aggregate to form cracks on parallel surfaces, and the cracks expand to produce wear debris and become peeling pieces. The WC hard phase also increases accordingly, and Fe3B hard phase is formed during the spraying process. The wear mechanism of the coating is mainly brittle exfoliation of the hard phase and slight plastic cutting. Under the condition of abrasive wear, on the one hand, due to the coating The metal matrix with lower medium hardness wears first, exposing the WC and Fe3B hard phases with higher hardness, the anchoring effect of the matrix on the hard phase decreases, and the high hardness WC and Fe3B hard phases are peeled off from the matrix under quartz sand cutting; On the other hand, due to the existence of microscopic pores and oxidized areas around the hard particles in the coating, the interior of the coating is relatively loose, and the water stored in the worn pores generates a high pressure under the action of the load, which makes the pores expand, connect, and finally The coating particles around the micropores are peeled off, causing wear.



The reasons for the high wear resistance of the coating are: (1) Many WC and Fe3B hard phases are anchored in the coating, which can block the expansion of ploughing during the abrasive wear process, resulting in the wear scars in the hard phase. (2) The dispersed WC and Fe3B hard spots in the FeCrNiB coating matrix make the matrix produce dispersion strengthening, the increase of hard phases will inevitably increase the grain boundaries in the coating, and the matrix will produce grain boundary strengthening, these strengthening effects make the coating The Rockwell hardness of the surface of the layer increases with the content of WC and Fe3B hard phases, which improves the micro-cutting effect of the matrix to withstand and resist hard abrasive particles and reduces the plastic deformation of repeated pushing. The number of hard points increases, the wear surface tends to be smooth, only the hard phase peels off pits, and there is no furrow due to plastic cutting. Since there are too many WC hard phases remaining in the 4# coating, the porosity increases, and the strength of the matrix increases. The toughness also decreases due to the increase of the brittle phase. Under the action of hard abrasive particles, microcracks are generated and propagated at the anchoring bond between the massive hard phase and the matrix, and when the length of the microcrack exceeds the critical dimension of the matrix fracture strength, wear debris fragments are formed In addition, with the increase of WC content, the unmelted WC particles increased during the spraying process, and the anchoring effect of the matrix gradually decreased, and it was easy to crack and break away from the matrix under the repeated pushing of sharp-angle hard abrasive particles.



3 Conclusion



a. Under the condition of abrasive wear, the wear resistance of the WC-containing ceramic phase coating is better, which is about 9 times higher than that of the Q235 steel; when the WC content does not exceed 25%, with the increase of the WC content, the WC ceramic coating The wear resistance of the coating increases, and when the WC content exceeds 25%, the wear resistance of the coating decreases.



b. In the arc sprayed WC-containing ceramic phase coating, the metal matrix with lower hardness wears first, and the WC and Fe3B hard phases with higher hardness play a role in preventing the wear of quartz sand, thereby reducing the wear of the coating.



c. In the WC-containing ceramic coating, the WC and Fe3B hard points dispersed in the matrix play a role in strengthening the matrix, so that the Rockwell hardness of the coating surface increases with the increase of the WC and Fe3B hard phase content, so Improve the ability of the substrate to withstand and resist micro-cutting of hard abrasive particles and repeated pushing and plastic deformation.