EP0455812A1

EP0455812A1 - Method for gas-plasma spraying of metal coatings

Info

Publication number: EP0455812A1
Application number: EP90913571A
Authority: EP
Inventors: Mikhail Valentinovich Karasev; Valery Stepanovich Klubnikin; Georgy Konstantinovich Petrov
Original assignee: Leningradsky Politekhnichesky Institut
Current assignee: Leningradsky Politekhnichesky Institut
Priority date: 1989-12-01
Filing date: 1990-01-15
Publication date: 1991-11-13
Also published as: WO1991008321A1; SU1835865A1; AU6331490A

Abstract

A method for gas-plasma spraying of metal coatings provides for feeding, accelaration and heating of metal powder particles (5) in the plasma jet and placing the surface of the articles (7) to be coated on the path of the particles (5), the dimensions of the particles (5) introduced to the jet being no less than 20 µm. The spraying of coatings is carried out at the enthalpy of the plasma jet (3) defined by the following relationship: H = (0.46 - 0.69)√d, where H is the enthalpy of the plasma jet (3) in kJ/g, d is the average dimension of the sprayed particles (5); the plasma-forming gas consumption is maintained at a value not exceeding that corresponding to the starting point of sedimentation of the coating (8) at the enthalpy of the plasma jet (3) H = 0.69√d, whereas the power of the plasma jet (3) is maintained at a value corresponding to the starting point of sedimentation of the coating (8) at enthalpy H = 0.46√d.

Description

Field of the Invention

The present invention relates to the field of plasma treatment technology and, more specifically, to plasma spraying processes used to produce metal coatings and using compressed air as plasma-forming medium. The invention can be used by any industry to obtain both general- and special-purpose coatings, preferably for hardening and reconditioning machine components.

Prior Art

The conventional plasma spraying methods using for plasma-forming media such inert gases as argon, helium, etc. as well as their mixtures with nitrogen, are capable of providing a sufficiently high quality of deposited metal coatings. However, when spraying is performed in an atmosphere of air, the plasma jet formed of one of the aforementioned media will change its gas composition with distance from the plasma generator nozzle. Thus, at the nozzle edge the plasma jet is formed entirely of the working fluid, while at increasing distances from the nozzle edge there is increasingly vigorous entrainment of atmospheric air by the jet (A. Hasui and O. Morigaki. "Naplavka i napylenie", 1985, The Mashinostroenie Publishers, Moscow, p.145). This tends to change the spraying process considerably, causing it to approximate in a measure the air plasma spraying process. (V.S. Klubnikin et al. "Promyshlennoe primenenie protsessov vozdushno-plasmennogo napylenia pokryty", 1987, the LDNTP Publishers, Leningrad, p.24).
Air plasma spraying technology relates to the new route of development in spraying technology, i.e. to plasma spraying in active media, which has presently gained a wide scope of application. The active media used include air, air and natural gas mixtures, carbon dioxide and natural gas mixtures, water vapour, etc. A common feature of all these gas media is the presence therein of chemically active oxygen. However, the presence of chemically active oxygen will limit the use of plasma spraying of metal powders because of plasma chemical interaction processes between plasma and metallic material. At the present time there are no systematic data available that would reflect in quantitative terms the nature of interaction between air plasma jet and metal. At the same time the oxidation of metal powders is known to lead to the burning-out of alloying components from the alloy being sprayed, to lower cohesive strength of the coating, and to changed chemical composition and properties (A. Hasui and O. Morigaki. "Naplavka i napylenie," 1985, The Masinostroenie Publishers, Moscow, p.155). There are a number of interrelated factors that affect the chemical composition and extent of oxidation of the coating obtained by air plasma spraying of metal powders, viz.: metal particle spraying rate, metal particle size and temperature.
As is known, control of metal coating oxidation may be effected by suitable selection of air plasma spraying process parameters, such as powder particle size distribution, plasma jet enthalpy, plasma jet power, plasma-forming gas flow rate, and others (V.S. Klubnikin et al. "Promyshlennoe primenenie protsessov vozdushno plazmennogo napylenia pokryty", 1987, The LDNTP Publishers, Leningrad, p.15).
Specifically, oxidation of metal powders in air plasma will increase with lower particle size and lower particle velocity, therefore in laminar plasma flow the burning loss of alloying components and the extent of oxidation of particles is considerably higher than in turbulent plasma flow. However, no specific relations of air plasma spraying process parameters, such as would assure the obtainment of low-oxidized metal coatings, are presently available. Spraying process parameters are largely dependent upon plasma generator operating parameters, such as plasma generator arc current, arc voltage, and plasma-forming gas flow rate. Selection of optimal process parameter values, such as would assure the obtainment of low-oxidized coatings, is fairly complicated and labour-consuming owing to the large range of variations in the interrelated plasma generator process parameters.
There is a known plasma spraying method comprising the steps of introducing powder particles to be sprayed into a turbulent plasma jet, accelerating and heating them therein, and placing the component surface to be coated in the path of particle flow (JP, A, 57-126964). Formation of the plasma coating occurs by formation of fusion (weld) areas over the contacting surfaces of deformed particles within the coating. This is achieved by selecting the spraying distance such that molten particles should be deposited over the component surface at their melting temperature, and the coating, as well as the component surface, should not be overheated.
In the method according to JP, A, 57-126964, the coating quality is enhanced by optimizing the particle size of the powder to be sprayed. It will be considered that the method here is one of plasma spraying in inert gas and that the coating properties implied are those of density and adhesive strength.
However, selecting particle size as the only air plasma coating process variable will not ensure minimal coating oxidation since the extent of oxidation of individual particles and of the entire coating, respectively, is affected by particle velocity and particle temperature, as previously noted.
There are known plasma spray coating methods, in which the coating quality is controlled by varying the operating variables of the plasma generator, i.e. arc current, arc voltage, and plasma-forming gas flow rate (FR, A, 2526331; DE, A, 3216025). Said methods are likewise designed for plasma spray coating based on the use of inert gases, by preference. In this case, the recommendation is for the maximum arc current and plasma-forming gas flow rate values for the plasma generator used in order that high-quality coatings might be obtained (regarding, specifically, the desired density and adhesive strength). This is based on the condition that the coating density and adhesive strength will increase with increasing particle velocity and increasing particle temperature, with the particle temperature preferably exceeding the melting point of the particles. It is, however, known that the temperature, to which a particle will heat in a plasma jet, is proportional to the enthalpy of the plasma jet, which is determined by the ratio of the jet heat output to the gas flow rate, so that arbitrary changes in said operating variables of the plasma generator may lead to particles heating up to a temperature exceeding their melting point and to the increased concentration of oxides in the coating. Besides, in said method, there is no correlation between the plasma jet parameters and the particle size of the material to be sprayed, this also resulting in an increased concentration of oxides in the coating applied.
There is known an air plasma spraying method to produce metal coatings, which comprises the steps of introducing metal powder particles into a turbulent plasma jet, accelerating and heating them therein, and placing the component surface to be coated in the path of particle flow (DE, A, 3435748).
The plasma spraying process parameters to provide the specified coating properties are determined experimentally as follows. Sprayed particle velocity and temperature are measured at the coating location, the quality of the coating formed is investigated, and the plasma spraying process parameters (current arc, plasma-forming gas flow rate, plasma jet power, particle size, etc.) are optimized in accordance with the results obtained. This method is preferably designed for spraying in inert gases, therefore the coating is investigated for such properties as adhesion, porosity, hardness, and cohesion whereas the oxidation of the coating is not considered. In this connection, the process variables are optimized based on the particles molten state which can be realized within a wide range of temperatures - from the melting point to the evaporation temperature. That is to say, in this technical solution, just as in the previous one, selection of optimal process parameters - specifically, arc current, plasma-forming gas flow rate - is effected in a fairly wide range. In this case, powder particles may overheat in the plasma jet and within the plasma for a comparatively long time, which in the case of air plasma spraying will lead to an elevated concentration of oxides in the coating. Besides, the process of experimental selection of optimal air plasma spraying process parameters as described above is quite labour-consuming and complicated.

Disclosure of the Invention

The present invention is based upon the objective of providing an air plasma spraying method wherein the spraying process parameters would be so interrelated and their limits would be determined such that a minimal concentration of oxides would be ensured in the coating formed, thus enhancing the quality of the coating and simplifying the process.
The objective as stated above is achieved by providing an air plasma spraying method to produce metal coatings, comprising the steps of introducing metal powder particles into a turbulent plasma jet, accelerating and heating them threin, and placing the component surface to be coated in the path of particle flow wherein, according to the invention, metal powder particles are introduced of a size not lower than 20 µm while coatings are applied at a plasma jet enthalpy determined from the relation H = (0.46 - 0.69) √d, where H is the plasma jet enthalpy, kJ/g, d is the mean size of the particles sprayed, µm, the plasma-forming gas flow rate being maintained not higher than at the level corresponding to the commencement of coating deposition at an enthalpy of H - 0.69 √d, and the plasma jet power not lower than the value corresponding to the commencement of coating deposition at an enthalpy of H - 0.46 √d.
The range of plasma jet enthalpy values, the maximum and minimum allowable values for plasma-forming gas flow rate and plasma jet power, respectively, and said limitation on the minimum size of particles to be sprayed, combine to define the boundaries of the proposed region of air plasma spraying process parameters to produce metal coatings. As found by the inventors experimentally, the process parameters (plasma-forming gas flow rate, plasma jet power, and sprayed particle size), whose values correspond to the proposed region, are capable of producing low-oxidized coatings using any metal powders. In the opinion of the inventors, the explanation lies in the fact that the region as defined by the inventors, is close in process conditions to the commencement of coating formation. The deposition of particles over the component surface in this case and the formation of the coating occur with the metal particles being heated to a temperature not exceeding practically their melting point and having the maximum possible velocity. For low-melting powders, the region of process parameters to provide for the formation of a low-oxidized coating is shifted towards higher gas flow rate and higher plasma jet power while for high-melting materials it is shifted towards lower gas flow rate and lower plasma jet power. Increasing the size of powder particles to be sprayed will expand the region of allowable process parameters. This is due to the extended range of plasma jet enthalpy values. In all events, the obtainment of a low-oxidized coating requires the plasma-forming gas flow rate to be sufficient to provide for turbulent plasma flow.
The proposed region of spray coating parameters and, specifically, the optimal plasma generator operating parameters are determined by a design-and-experimental method, in contrast to the method according to DE, A, 3435748. Using a design-and-experimental method will reduce the number of technological experiments and lead to a lower level of labour consumption involved in the method.
Optimal air plasma spraying process parameters for a specific metal powder spraying process are selected from said region, using previously known procedures, and depending on the desired quality of the coating to be obtained (apart from oxidation) and on the required throughput rate of the spraying process.
It has been found by the inventors experimentally that at process parameters (plasma-forming gas flow rate and plasma jet power) other than those corresponding in value to the proposed region, either no plasma coating will be deposited and formed or an oxidized coating/a coating with unstable properties will result. Another experimental finding is that using sprayed powder with a particle size of less than 20 µm will sharply increase the oxygen concentration in the coating to reach considerable levels.

Brief Description of the Drawings

The aforesaid advantages, as well as features, of the present invention will become more clearly understood upon further studying the following detailed description of the best mode of carrying the invention into effect with due references to the accompanying drawings, wherein:

Figure 1 illustrates a flow diagram of air plasma spraying process to produce metal coatings; and
Figure 2 shows the isometric lines of enthalpies H₁ = 0.46 √d and H₂ = 0.69 √d, as well as the process parameter region to provide for the formation of low- oxidized coatings.

Best Mode of Carrying out the Invention

In accordance with the invention, the air plasma spraying method to produce metal coating is implemented as follows. The exit section of the nozzle 1 (Figure 1) of a plasma generator (not shown) serves to form an oriented air plasma stream 2 which forms a turbulent plasma jet 3 at exit from the nozzle 1. The gas flow rate required to create turbulent flow is determined by known methods (Inkropera and Lepert, Yavleniya perekhoda techneniya v dozvukovoi plazmennoi strue". Raketnaya Tekhnika i Kosmonavtika, 1966, No.6, pp.164-165).
Through a gas and powder channel 4, sprayed metal powder particles 5 are fed into the plasma jet 3, the turbulent jet 3 serving to accelerate and heat up said particles. The sizes (d) of the particles 5 are selected by known methods, in relation to the required density and adhesive strength of the coating (V.S. Klubnikin, "Plazmennye ustroistva dlya naneseniya pokryty", Izvestiya Sibirskogo Otdeleniya Akademii Nauk SSSR, 1983, No.13, vol.2 (Novosibirsk), pp.82-92). However, the size of the particles 5 should never be less than 20 µm. This requirement is due to the fact that, as found experimentally by the inventors, spraying metal powder with a particle size less than 20 µm will sharply increase oxygen concentration in the coating to fairly high levels. While particles 5 move within the plasma jet 3, an oxide film 6 is formed over said particles 5. Sprayed metal powder particles having a transverse dimension d and a velocity v are heated within the plasma jet 3 to a temperature T. The temperature T of the particles 5 is maintained within the plasma jet 3 equal to the melting temperature of the metal powder sprayed while the velocity v of the particles 5 is maintained at the maximum level providing for this condition. On reaching a component 7 placed in their path of flow, the particles 5 are deposited on its surface, causing a coating 8 to form thereupon. Spraying distance is selected by known methods, based on the requirement for molten particles 5 to be deposited on the surface of the component 7 at their melting temperature and for the coating 8 and the surface of the component 7 to be prevented from overheating (A.V. Donskoy and V.S. Klubnikin, "Elektroplazmennye protesessy i ustanovki v mashinostroenii", 1979, The Mashinostroenie Publishers, Leningrad, p.192; A. Hasui and O. Morigaki, "Naplavka i napylenie", 1985, The Mashinostroenie Publishers, Moscow, p.173). Overheating of the coating 8 will generally lead more especially in air plasma spray coating, to their vigorous oxidation. Where high-melting metal powders are used, i.e. where the temperature is sufficiently high before the powder material begins to be oxidized vigorously, the surface temperature of the coating 8 and the component 7 - such that would preclude their overheating - is maintained by suitably selecting the spraying distance. While using low-melting metal powders, when a relatively slight overheating of the coating 8 will initiate vigorous oxidation thereof, the component 7 is additionally cooled by, e.g. compressed air.
The temperature of the particles 5 within the plasma jet 3 equal to the melting temperature of the material of the particles 5 and the maximum possible velocity of the particles 5, consistent with this temperature, will provide minimal oxidation of particles 5 within the jet 3. It has been found by the inventors experimentally that, depending on the melting temperature of the material of the particles 5 and other thermal parameters within the range of enthalpy values of H = (0.46 - 0.69) √d, there exists a certain region of process parameters (arc current power of the plasma jet 3, plasma-forming gas flow rate), wherein there is no overheating of powder particles 5 substantially higher than their melting temperature over the path of the particle flow towards the component 7. That is to say that any air-plasma metal-powder spraying process parameters out of said region will provide for the formation of a low-oxidized coating. Owing to a predetermined and fairly narrow region of air plasma spraying process parameters being specified to provide for the formation of a low-oxidized metal coating, said optimal parameters are selected in each specific spray coating process in a comparatively simple way, following known procedures and based on the required coating quality (apart from the extent of oxidation) and process throughput rates.
Determining the boundaries of said process parameter region is by means of a design-and-experimental method which can be illustrated with the help of Figure 2. In accordance with the required coating quality (density, adhesive strength), the size d of sprayed powder particles 5 is selected (not to be less than 20 µm), and numerical values are determined for the enthalpies H₁ = 0.46 √d and H₂ = 0.69 √d. This done, isometric lines plotted for the enthalpies H₁ and H₂ in the coordinates P and G from the relation H = P/G, which lines in said coordinates are straight lines with coefficients of proportionality H₁ and H₂. The plasma generator arc current corresponding to the values of P and G which are necessary for obtaining calculated enthalpy values, are calculated using the known relationship:

where: I = arc current, A; G = plasma-forming gas flow rate, g/s; D = plasma generator arc channel diameter, cm; L = plasma generator arc channel length, cm (V.S. Klubnikin et al. "Promyshlennoe primenenie protsessov vozdushno-plasmennogo napyleniya pokryty". 1987, The LDNTP Publishers, Leningrad, pp..8-9).
Thus, calculations are used to determine arc current and plasma-forming gas flow rate values for the plasma generator, such that are required to obtain the values of enthalpies H₁ and H₂ and that of plasma jet power P. In the experimental part, process parameters are determined, corresponding to points A,B, and D which define the limit values of said parameters. This is achieved by determining plasma-forming gas flow rate G^I and plasma jet power P^I which correspond to the commencement of coating deposition at H₁ = 0.46 √d (point A). Said values are determined by way of spray coating several specimens, with the arc current and gas flow rate being decreased against the maximum possible values for the given plasma generator, prior to the commencement of deposition of coating 8 over the surface of the component 7 at enthalpy H₁. A similar procedure is used to determine plasma-forming gas flow rate G^II and plasma jet power P^II corresponding to the commencement of coating deposition at H₂ = 0.69 (point B). Point D is determined as the point of intersection of line H₂ and value P^I. Next, a curve 9 is plotted to define a region 10 where the material used, in powder form, is deposited by way of air plasma spraying. The curve 9 is formed as a number of points corresponding to the commencement of coating deposition based on the material selected, in powder form, at various plasma jet enthalpies and determined by means of standard technological experiments similar to those used for determining points A and B. As a result, the process parameter region 11 characterized by the formation of low-oxidized coating, will be defined by the isometric lines H₁ and H₂, gas flow rate G^II, plasma jet power P^¹, and powder spray coating curve 9 to give region of optimal process parameters. In Figure 2, the region 11 is shown in the form of a figure having vertices A,B, and D. A region 12 comprised in the powder spray deposition region 10, is a process parameter region which is characterized by the formation of oxidized coatings. With the process parameter values beyond the optimal process parameter region 11 but within the enthalpy range of H = (0.46 - 0.69) √d, a coating will either fail to be deposited at all (right-hand region), or will have unstable properties (lower part). In the latter case, the oxygen concentration in the coating will vary over a wide range. Oxidation of coatings in this process parameter region is due to decreasing particle velocity at constant enthalpy (within H = (0.46 - 0.69) √d. This is associated with decreasing gas flow rate and arc current values. The residence time of particles within the plasma jet is increased in this case, and their vigorous oxidation may be initiated. The process parameter region defined by isoline H₁ and curve 9, is characterised by deposition of coating which are lower in cohesion, adhesion, density and other properties. This is due to the low arc current failing to provide for an optimal combination of particle velocity and particle temperature and to the formation of a coating having fused sections between particles. In said region, formation of coating occurs mainly by mechanical bonding between particles and substrate and by their joint plastic deformation. In certain case, plasma coatings may form within this region of process parameters, but their properties are instable due to low particle velocities. The region between the lines H₁ and 9 is very small in size and, in several instances, equal to zero.
With the process being carried out within the region 12, above the isoline H₂, a strong plasma chemical interaction between particles and plasma will lead to vigorous particle oxidation at any process parameters.
In plasma spraying of low-melting powders, e.g. tin with a melting temperature (T) of 232^oC and a thermal conductivity (λ) of 73.2 W/m K, the optimal process parameter region 11 is shifted towards higher gas flow rates. The commencement of coating deposition at H₂ = 0.69 √d (intersection of isoline H₂ and curve 9) is achieved in this case at substantial gas flow rates (point G¹ is shifted to the right). When spraying high-melting powders (e.g. tungsten, T = 3400^oC,λ = 174 W/m K), the commencement of coating deposition at H₂ = 0.69 √d (intersection of isoline H₂ and curve 9) is shifted towards lower gas flow rates. The optimal process parameter region 11 is narrowed in this case as compared to the case of low-melting powder spraying. When spraying powders with a medium melting temperature (e.g. iron, T = 1536^oC, λ = 78.2 W/m K), the optimal process parameter region occupies an intermediate position.
It will be noted that increasing powder particle size will increase the optimal process parameter region 11. This is due to the increasing value of (H₂-H₁). It will be considered that in all cases the selected plasma-forming gas flow rate must be sufficient to assure turbulent plasma jet flow. Owing to the proposed region 11 of spraying process parameters being determined by a design-and-experimental method, there will be fewer technological experiments required than in the method according to DE, A, 3435748, this making the method less labour consuming.
The air plasma spraying method to obtain metal coatings, as described schematically above, was used to spray coat specimens using various metal powders. The spraying process parameters, powder materials, and results of coating quality investigations are given in Tables 1 and 2. Table 1 cites specific spraying process parameter values and coating oxidation estimates. The selection of said parameters corresponded to points A,B,C, and D in the process parameter region 11 illustrated in Figure 2 where point A corresponds to the commencement of coating deposition at the enthalpy of H₁ = 0.46 √d, point B to the commencement of coating deposition at the enthalpy of H₂ = 0.69 √d, point C is between points B and D on the straight line H₂, and point D is the point of intersection of the value of P^I and the straight line H₂.
In the region of each of said points, spraying was performed at different process parameters providing for three H/√d values.
The extent of coating oxidation was estimated by way of visual inspection. The powders used for spray coating were those of tungsten with particle sizes equal to 86 µm, 21.3 µm, and 19.3 µm, those of tin (d = 86 µm and 19.3 µm), and nickel-chromium-boron-silicon alloy (d = 26.1 µm), 19.3 µm, and 20 µm). At the same time, a sampling analysis was performed for oxygen concentration in coatings obtained from the following powders: nickel- aluminium, iron-based alloy, and nickel-chromium-boron- silicon alloy with an addition of tungsten (Table 2). Analysis for oxygen concentration was performed in a Leko unit (USA), and powder particle size analysis in a Gilas unit. For coating purposes, an air plasma generator was used, with an arc channel length of 35 to 53 mm. A bank of DC sources was used as a power supply unit. Also, a powder meter was utilized for powder feeding. The spraying distance was selected equal to 140-200 mm.

Spraying nickel-chromium-boron-silicone alloy, tungsten and tin powders with a particle size of 19.3 µm has demonstrated that an oxidized coating is formed whatever the spraying conditions.
Analysis of the data cited in Tables 1 and 2 shows that process parameters with values corresponding to the proposed region 11 (Figure 2) will provide for the formation of low-oxidized metal coatings obtained by air plasma spraying techniques. Oxygen concentration in such coatings is close to that in similar metal coatings obtained by argon plasma spraying. Thus, oxygen concentration in a coating produced from nickel-chromium-silicon alloy powder by argon plasma spraying is between 0.05 and 0.15% while in a coating based on nickel-aluminium alloy powder it is equal to 0.1% (V.S. Klubnikin et al., "Promyshlennoe primenenie protsessov vozdushno-plazmenogo napylenia", 1987, The LDNTP Publishers, Leningrad, p.15). It can be seen from Table 2 that oxygen concentration in metal coatings obtained by air plasma spraying at process variables corresponding to the proposed region 11 lies within the same range of values (Examples 3 to 6, 9 to 11, 14 to 17). For instance, the oxygen concentration in a nickel-aluminium alloy coating does not exceed 0.1% either (Examples 3 to 6 in Table 2). In the case of air plasma spraying of metal powders at process variables with values beyond the boundaries of region 11, there is either no coating formed, or a coating is formed with a high oxygen concentration or with unstable properties. Specifically, in the region 12, oxidized coatings will be formed Examples 6,9,11,12,18,21,23,24, etc. in Table 1 and Examples 7,12, and 18 in Table 2). No coatings will be deposited beyond the boundaries of region 10 (Examples 2,5,14,17,26, etc. in Table 1).

Industrial Applicability

The proposed method can be used to best advantage in the application of metal coatings, preferably for hardening and reconditioning machine components.

Claims

An air plasma spraying method to produce metal coatings, comprising the steps of introducing metal powder particles (5) into a turbulent plasma jet (3), accelerating and heating them therein, and placing the component surface to be coated in the path of particle flow, characterized in that the metal powder particles (5) are introduced of a size not lower than 20 µm, while coatings are applied at an enthalpy of the plasma jet (3) determined from the relation H = (0.46 - 0.69) √d, where H is the plasma jet enthalpy, kJ/g, and d is the mean size of the particles (5) sprayed, µm, the plasma-forming gas flow rate being maintained not higher than at the level corresponding to the commencement of deposition of the coating (8) at an enthalpy of H = 0.69 √d, and the power of the plasma jet (3) not lower than the value corresponding to the commencement of deposition of the coating (8) at an enthalpy of H = 0.46 √d.