DURABILITY OF THE LAYERS DEPOSED WITH METAL CERAMIC POWDERS TYPE Cr 3 C 2 NiCr AND HIGH SPEED FLAME (HVOF)

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1 9 DURABILITY OF THE LAYERS DEPOSED WITH METAL CERAMIC POWDERS TYPE Cr C NiCr AND HIGH SPEED FLAME (HVOF) Doina PETRESCU, Niculae Napoleon ANTONESCU Petroleum-Gas University Of Ploiesti, ROMANIA dpetrescu@upg-ploiesti.ro ABSTRACT The durability of the reconditioned or preventively covered with powders type Cr C NiCr product may outrun the performances of the original product and (the original specification, too). Due to the deposit of these layers with superior properties of the basic material, there may be maintained also at a maximum level maintenance expenses during a long period of time. In the present study there are studied the effects of the main high speed flame thermal spraying parameters on some properties of the layers deposed with powder type Cr C NiCr, with the help of the orthogonal regression method and also the correlation of the deposits properties. Keywords: durability, adhesion, hardness, HVOF. INTRODUCTION The assembly scheme of the deposit by high speed flame thermal spraying (HVOF) installation may be observed in Figure. The technology of the deposit with high speed flame is based on the utilization, as a powder wearer agent, of some hot gases (their leakage sp eed is of approximately.5 m/s, at a pressure easily higher towards the atmospheric one) []. The powder particles during the spraying process suffer only a tempering not melting that ensures a reduced oxidation and a minimal modification of the chemical composition in the deposed layer. The procedure of high speed flame deposit is used mostly to the production of the dense coverings with cermet powders (especially), that have a superior adhesion and a resistance to wear. Anyway, the properties of the layer deposed with Cr C NiCr will be affected by the spraying conditions. In the specialty technical literature there are many observations concerning the effect of some technological parameters on the deposits properties but there are few systematic registered rapports that investigate the main spraying parameters (oxygen volume, propane volume and spraying distance). The technology of depositing by thermal spraying has an empiricism degree high enough due to the phenomena complexity that take place during the deposit process and due to the over 50 control parameters and also due to the fact that there is not known a satisfactory mathematics relation between the spraying parameters and the deposed layer microstructure. Fig.. The assembly scheme of the high speed flame thermal spraying However, by the correct choice of the thermal spraying depositing parameters there are obtained layers of the highest quality with multiple practical utilizations. In the present study, there are discussed the effects of the main high speed flame thermal spraying parameters on some properties of the layers deposed with powder type Cr C 5%NiCr, with the

2 9 Table. Spraying parameters Coefficients -r r Oxygen volume,, (slpm*) Propane volume,, (slpm*) Spraying distance, x0 - m regression method and the correlation of the deposits properties. The granulation of the used powder (Cr C 5%NiCr) is between 0 6 μm. In order tot measure the adhesion force and the abrasive wear there was used as a sub-layer faint allied steel (Cr -0.5Mo, JIS-G46--STBA). Before the thermal spraying, the sub-layer was sand blasted with quartz sand. When the spraying distance (parameter ) was varied (according to Table ), the oxygen and propane pressures were fixed at N/m and 0.4 x0 N/m. In Table there are shown the values of the three main thermal spraying parameters (, - oxygen, propane volumes and spraying distance). In Table there are presented the combinations of the spraying parameters for the 5 value sets (of spraying conditions). Table. Arrangement of the 5 values sets No. Oxygen volume,, (slpm) Propane volume,, (slpm) Spraying distance,, (x0 - m) r r r 0 0 -r r r The experiment was made according to the standard ASTM6-79 and the experimented mark diameter was of mm. The average adhesion was calculated as the arithmetic average of the 5 marks. The micro-hardness of the deposed layer was measured at a task of 00 gf in0 points (on diagonal) in the section of the sub-layer. The wear of the deposed layer was experimentally determined with the help of the stand diagrammatic presented in Figure. There was used as abrasive material the aluminum oxide powder (with the granulation of 0μm) according to ASTM- G65-9. The test was made with a loading of 0 N, at a speed of 50 rev/min, during 5 minutes. The deposit microstructure was studied using optic microscope and the electronic scan. Fig.. The stand for the resistance to wear determination.. The Adhesion of the Layer Thermally Sprayed with High Speed Flame The adhesion of the layers thermally sprayed with powder Cr C NiCr deposed with high speed flame varied from 5-76 x0 N/m (Table ) since the change of the thermal spraying conditions. The real adhesion of such layers is bigger than the one experimentally determined, ]. Y σ is the adhesion of the layer at the sub-layer, Y h - the hardness of the deposed layer, Y w - the resistance to the abrasive wear of the deposed layer. Y, Table. Parameters experimentally determined No. Y σ, Y w, in x0 N/m h in HV 0. in 0-6 Kg 60.7* * * * * * * * cracking occurrence during the test.

3 94 The regression formula for the adhesion of the deposed layer, when there are varied the three technological parameters, respectively, the oxygen volume, the propane volume and the distance of thermal spraying is the following: Y = () The equation verification () with the test F from ASTM-G65-9 shows that there is obtained: F =.64 > F α = 0.0 (5.9) =.6 () According to the regression formula () it results the oxygen volume influence less the adhesion of the thermally sprayed layer (the oxygen i s introduced anyway in excess. O ), but the propane volume and the distance of the thermal spraying consistently influence the depositing adhesion. From Fig. it may be observed that the adhesion of the thermally sprayed layer increases with the increase of the propane volume. It may be observed from Fig. 4 that at a certain propane volume, the adhesion increases with the increase of the spraying distance till a maximal value of 0.80 m and then decreases while this distance increases. influence on the hardness of the thermally sprayed layer. Fig... The Hardness of the High Speed Flame Thermally Sprayed Layer and Powder Type Cr C 5%NiCr The covering average hardness, as it results from Table varied from 55 HV to approximately 900 HV since the change of the spraying parameters. The regression formula for the relation between the covering hardness and the spraying parameters ( oxygen volume, propane volume and the thermal spraying distance) that emphasized significant effects on the hardness of the thermally sprayed layer is the following: Y = h () The test F applied the regression formula () has the following results: F = 4.65 > F = 0.05 (7.7)=.75 (4) Analyzing the regression formula () it may be deduced that the hardness of the layer deposed with high speed flame decreased with the increase of the spraying distance (for certain propane volumes). In Figure 5 it may be observed the influence of the propane volume on the hardness of the thermally sprayed deposed layer at an optimal spraying distance of 0.0 m (see Fig. 4) and for different oxygen volume. When the propane volume outruns 0 slpm, the layer s hardness increases with the increasing of the propane volume. Also, from Fig. 6, it may be noticed that the oxygen volume has a limited Fig. 4. Fig. 5.

4 95 Fig. 6.. The Influence of the Abrasive Wear on the High Speed Flame Thermally Sprayed and Powder Type Cr C NiCr The loss in weight (see Table ) due to the abrasive wear of the metal ceramic layers is evidently influenced by the thermal spraying parameters ( the oxygen volume, propane volume and thermal spraying distance). The regression formula obtained for the loss in weight due to the abrasive wear is the following: Y = w (5) The test F applied to the regression formula ( 5) leads to the following results: F = 4.0 > F = 0.05 (7.7)=.79 (6) From Figure 6, it may be observed the effect of the thermal spraying distance on the abrasive wear at an oxygen volume of 440 (in slpm) and at different propane volume (according to the relation 5). It may be observed that the thermal spraying distance depends on the propane volume, thus at a small volume, the loss in weight due to the abrasive wear of the deposed layer increases in the same time with the increase of the thermal spraying distance. In the conditions of a high propane volume, the abrasive wear of the deposed layer is not meaningfully influenced by the thermal spraying distance. In Fig. 7 it may be noticed the effect of the propane volume on the loss in weight due to the abrasive wear of the deposed layer with different oxygen volume at the thermal spraying distance of 0.0 m. The concave curve resulted from the increase of the propane volume, thus the diagram shows the evidence of the fact that the sub-layer with a decreased loss in weight due to the abrasive wear should be deposed at a certain propane volume. The effect of the oxygen volume on the mass loss (of the deposed layer) by abrasive wear has the same tendencies as in the case of the propane volume as it is presented in Fig The Correlation of the Deposed Layer Properties The materials hardness is generally given by the mechanical properties especially by the resistance to wear. The bigger the materials hardness is, the higher the mechanical characteristics are. For the thermal sprayed layers, due to the measurement facility, the micro-hardness gives information on the mechanical properties of the deposit. Fig. 7 Fig. 8 Fig. 9

5 96 Fig. 0 With the help of the experimentally obtained data, previously presented, there may be studied the correlations between the deposed material hardness and the mechanical properties of the layer. In Figure 9, it may be observed the dependency of the adhesion on the deposed layer hardness and it may be drawn the conclusion that the adhesion of the deposits with powder type Cr C NiCr increases in the same time with the increase of the layer hardness. The connection between the adhesion and the deposed layer hardness (Fig. 9) could be suggested by the fact that the hardness of some layers from powder type Cr C NiCr depends meaningfully on the cohesion between the particles (lamellar) existent in the layer. The layer deposed with powder type Cr C NiCr has a microstructure composite with chrome carbide particles distributed in the ally matrix NiCr, as it may be observed in Fig. 0. The particle s state, respectively solid of the carbide and fluid for the metal stage of the bonder, at the thermal spraying with high speed flame, should be associated with the high value of those deposits adhesion at the basic material. Continuing the comparative combination of the hardness and of the abrasive wear it is not found the existence of an evident relation between the two material characteristics, this is due to the fact that the wear of such layers obtained by composite powders will be strongly dominated by the carbide content, by the repartition /distribution and the size of the carbide particles and also by the combination of the carbides with the basic matrix. The hardness of the layer with powder type Cr C 5%NiCr could be affected mainly by the cohesion of the particle in the layer. 4. The durability of the preventively reconditioned /covered products depends on the properties of the deposed layers such as adhesion, hardness, resistance to abrasive wear that at their turn, these are influenced by the thermal spraying conditions, respectively by the main technological parameters (respectively the oxygen volume, the propane volume and the spraying distance). The spreading diagram is represented in a coordinate axle octagonal system on the basis of the points determined by the pairs of values that represent the technological parameters experimentally determined (oxygen volume, propane volume, spraying distance). The adhesion of the layer deposed with powder type Cr C NiCr is meaningfully influenced by the combustible gas volume, increasing with the propane volume. There are optimal gas volumes thus for the propane and also for the oxygen that could realize a layer with very good behaviors at the abrasive wear. The adhesion of the layers realized with powder Cr C NiCr tends to increase in the same time with the increase of the hardness. There is no evident relation between the hardness and the resistance to wear of the layer sprayed with the high speed flame. REFERENCES. Petrescu D., 008, Cercetări privind pulverizarea termică cu aplicaţii la recondiţionarea şi acoperirea preventivă a pieselor (in Romanian), PhD Thesis, Petroleum Gas University of Ploiesti, Ploieşti, Romania.. Petrescu D., 00, Cercetări teoretico-experimentale privind depunerile superficiale prin pulverizare termică (in Romanian), Petroleum Gas University of Ploiesti, Romania.. Li C.J. et al., 005, Effect of Spray Conditions on the Properties of HVOF Cr C NiCr Coatings, Proc. of a 4th International Thermal Spray Conference, Kobe, Japan, p Petrescu D., Antonescu N.N., 008, Experimental researches in order to obtain hard coatings by thermal spraying with high speed flame (HVOF), Journal of the Balkan Tribological Association, vol.4, no.4, pp Petrescu D., Antonescu N.N., 009, Theoretical and experimental researches concerning the adhesion of the layers deposited with alloys type NiCr by thermal spraying, The th International Conf. Tribology, Belgrade, Serbia, pp. 7-.