INTRODUCTION

Component surface preparation before coating plasma deposition is important in activating processes for bond formation between deposited particles and a base [1 – 7]. In this case property instability for a treated surface is a consequence of failure to observe the required treatment regime that often leads to coating preparation with poor adhesive properties.

Currently the following methods are used for preparing a base surface before coating application: jet-abrasive, mechanical (knurling, cutting with a “ragged tool”, etc.), thermal (preliminary surface warm-up), chemical (selective etching), electric spark treatment, and surface alloying in order to improve a base chemical activity [8 – 11]. The most widespread and promising method is jet-abrasive treatment [12 – 16] including action on a machined surface of a jet of hard particles moving at high speed. As a result of this there is formation at a surface of a new microrelief with a plastically deformed layer.

The main parameters governing component treated surface properties are [17 – 20]: form, size, shape of the particles used, their consumption per unit of treated surface, impact velocity with a base, and attack angle. During jet-abrasive treatment corundum, steel powder, quartz sand, cast iron or steel shot, “chopped” shot or steel wire lumps 0.3 – 1.2 mm in diameter, silicon carbide, graphite, etc., are used. The particle material used for surface preparation is selected in relation to its density, hardness, impact strength, particle shape and size, and also cost. In this case proceeding from the following considerations; a requirement for selecting such a material whose particles on impact over a treated surface do not break down or deform. The optimum particle size for any material should be of the order of 350 – 550 μm; particles should have an angular shape.

With use of corundum, quartz sand, and graphite consideration should be given to their low adhesive capacity with a surface since part of the material remains on a treated surface due to the content within it of readily broken minerals. Abrasive jet energy parameters, acting on a component material, may be controlled by changing a nozzle diameter and air injector diameter of a jet gun, compressed air consumption, distance from the nozzle outlet to a surface being treated, and treatment duration [21].

The aim of this work is optimization of a jet abrasive treatment regime recommended in preparing a component surface before plasma deposition in order to obtain high quality strongly adherent coatings.

RESEARCH OBJECT AND PROCEDURE

The method of abrasive-jet treatment for a component surface has been studied [22] on the example of a pit drill bit (Fig. 1). Cast iron grit with a size of 60 – 125 μm and silicon carbide grit with a size of 63 – 100 μm were used in order to prepare a surface for plasma deposition [23].

Fig. 1.
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Jet-abrasive treatment pit drill bit deposition.

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The main action on a treated surface is particle kinetic energy. In this case particle velocity V, m/sec, used for jet abrasive treatment was determined from an equation

$$ mV={C}_x\frac{\rho {V}_{\mathrm{g}}^2}{2}, $$
(1)

where m is particle weight, kg; Cx is hydrodynamic resistance coefficient; ρ is air density, kg/m3; Vg is gas velocity determined in relation to pressure and gun nozzle diameter, m/sec.

During a study of the effect of degree of work hardening on coating adhesive strength the variable of jet abrasive treatment was taken as the abrasive material specific consumption [9, 24], i.e., the weight consumption q, c/cm2, of treatment particles per unit of treated area that was determined by an empirical relationship

$$ q=\frac{3\pi {d}^2V\upalpha \uprho \upbeta}{4S}, $$
(2)

where d is jet abrasive gun nozzle diameter, mm; V is particle average velocity during jet abrasive treatment, m/sec; α is jet filling coefficient during jet abrasive treatment; ρ is particle material density, kg/m3; β is particle utilization coefficient; S is component treated surface area, m2.

In order to evaluate the amount of material removed during jet abrasive treatment specimens with a size of 100 × 100 × 3 mm were prepared from steel grade St3. Treatment was accomplished with a heat-abrasive tool with a pressure of 0.6 MPa [8, 25, 26]. The abrasive used was silicon carbide grit. The treatment distance was kept constant within the limits of 100 mm. Before and after treatment specimens were weighed on laboratory scales making it possible to determine the amount of metal removed [27, 28].

For quantitative evaluation the degree of roughness obtained was measured recording the treated surface profile on a chart strip of a polygraph-profilometer followed by result processing in accordance with GOST 19300–86 and GOST R 8.651–2009 by procedures adopted in engineering [29 – 31]. A polygraph-profilometer was used in order to obtain calibrated relationships for roughness property dependences on jet abrasive treatment regimes.

During selection of nozzle construction in accordance with recommendations [22] the equipment operating characteristics for the jet abrasive treatment provided in Tables 1 and 2 were used.

Table 1. Dependence of Compressed Air Consumption on Jet Gun Nozzle Diameter
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Table 2. Dependence of jet Gun Productivity on Diameter and Compressed Air Pressure
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Dependence of the amount of discharged grit on nozzle diameter is provided below:

Nozzle diameter mm . . . . . 4 6 8 10 11 12

Grit amount, kg/h . . . . . . . 560 1000 1500 1900 2500 3400

RESEARCH RESULTS

The actual surface area after jet abrasive treatment was determined for both the height and also the number of micro-projections. An increase in micro-projection height leads to an increase in temperature within the contact that worsens thermophysical conditions for occurrence of activated processes. In addition, an increase in surface roughness improves its adhesion capacity with deposited material from the point of view of “wedging” melt at surface micro-projections. An increase in the number of peaks increases the overall area of welding sections and also the overall contact area of deposited particles with component material.

With use of the same abrasive material with the same jet gun nozzle with an increase in compressed air pressure there is an increase in flushing process productivity. With an increase in abrasive material particle size flushing productivity increases although there is an increase in the distance between projections or cavities of a rough surface. Here it is expedient to use a mixture consisting of fine and coarse grains. With air pressure from 0.4 to 0.6 MPa the cast iron grit particle velocity with diameter of 0.34 – 0.71 mm varies within the limits of 40 to 60 m/sec.

The effect of treatment regime on a surface is different for materials with different hardness. Roughness Rz with particle diameter of 0.35 and 0.70 mm is provided in Table 3 from which it is seen that the particle size has a strong effect on micro-roughness height.

Table 3. Roughness Rz of Base Material with Abrasive Material Particle Velocity 50 m/sec
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For the prescribed air consumption, particle size and density there is some critical distance from nozzle orifice of a jet gun to a treated surface, exceeding which leads to a sharp reduction in particle velocity (Fig. 2). The critical distance increases with an increase in compressed air consumption, and particle size and density. Under very similar conditions the treatment efficiency decreases due to a screening effect of compressed air and particles reflected from the surface. With different distances of gun jet nozzle to a treated surface roughness generally changes differently with an overall increase that points to a requirement for experimental selection of the distance in each particular case.

Fig. 2.
figure 2

Dependence of abrasive particle average velocity Va on distance to nozzle exit L: ●) corundum particles (1.0 – 1.6 mm); ∆) cast iron shot (0.5 – 1.0 mm).

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With an increase in treatment duration roughness of a base changes insignificantly, and the degree of work hardening increases in proportion to treatment duration up to saturation.

Coating adhesive strength with a base with an increase in rate of material specific consumption (degree of work hardening increases significantly (Fig. 3) in spite of some reduction in base surface roughness. In our view and increase in work-hardened specimen adhesive strength should be related to the amount of interatomic bond formation. Some reduction in adhesive strength with a stable degree of work hardening may be explained by the fact that a deformed layer emits completely part of the energy that goes into the interatomic bond and entry of energy ceases, and a reduction in roughness leads to a reduction adhesive strength. An increase in abrasive material specific consumption leads to an increase in coating adhesive strength.

Fig. 3.
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Dependence of adhesive strength σad on abrasive material specific consumption q: ○) base 30Kh4G; ∆) base G13N4.

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The effect of particle diameter and velocity on depth and degree of base work hardening (Table 4) shows that the depth of a work hardened layer L depends on the same factors as surface roughness, and may be estimated approximately by an empirical relationship of the form

Table 4. Effect of Particle Diameter and Velocity on Depth L and Degree of Work Hardening η
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$$ L=\gamma Rz, $$
(3)

where γ is a coefficient depending on abrasive particle shape and properties of a surface being treated (γ = 4.2).

It is seen from Tables 3 and 4 that in order to obtain the optimum base parameters (Rz, L, η) treatment of a base material with hardness 170 – 230 HV should be conducted with particles up to 0.7 mm in diameter, and for a base with hardness 350 – 450 HV with particles 0.7 – 1.2 mm in diameter.

In determining the change in coating adhesive strength with a base in relation to production parameters for the deposition process a requirement arises for obtaining the prescribed level of surface roughness intended for plasma deposition. The roughness characteristics in question (height and amount of micro-roughness) are quite simply controlled since they are determined mainly by the size of abrasive grains and treatment duration. Specimen profilograms having a different degree of roughness are shown in Fig. 4.

Fig. 4.
figure 4

Profilograms for specimens with different degree of roughness Rz: a) silicon carbide grit, τ = 10 sec, Rz = 19 mm; b ) cast iron grit, τ = 10 sec, Rz = 32 μm; c) cast iron grit, τ = 35 sec, Rz = 55 μm.

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Results are shown in Figs. 5 and 6 obtained after treatment of profilograph-profilometer diagrams. General data for the change in steel St.3 specimen roughness parameters in relation to jet abrasive treatment regime are shown in Table 5. It should be noted that the value of n obtained by means of measurements in a profilograph-profilometer will possibly be somewhat lower than the actual amount of peaks since the smallest peaks and depressions are not recorded. However, for quantitative comparison the value of n obtained is of specific interest. For both forms of treated material n decreases with an increase in τ (see Fig. 6). It is seen that with use of cast iron grit the value of n changes by a factor of 1.8 more intensely than with treatment with silicon carbide grit. In this case after 30 sec of treatment with silicon carbide grit the value of n emerges into some constant level that is not observed during treatment with cast iron grit.

Fig. 5.
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Dependence of surface Rz on abrasive jet treatment duration τ and type of abrasive material: ○) Ra; ∆) Rz; □) Rmax; - - -) cat iron grit; ——)silicon carbide grit.

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Fig. 6.
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Dependence n for length 2.5 mm on τ and type of treating material: ○) cast iron grit; ∆) silicon carbide grit.

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Table 5. Specimen Surface Roughness Characteristics for Structural Steel in Relation to Abrasive Jet Treatment Regime
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For convenience of comparing the profile development obtained with treatment of a different area of surface we introduce a treatment intensity coefficient K whose value specifies treatment duration of a unit area of working surface.

Simultaneously with a reduction in n with an increase in τ surface roughness increases that is specified by the increased degree of plastic deformation of a treated surface. With a significant increase in τ, when K is more than 60 sec, the curves for dependences of Rz on τ have minimum values (see Fig. 4). This may be explained by development of work hardening and material removal from a surface with prolonged action of an abrasive jet.

The actual surface area of a specimen after jet abrasive treatment, that as we have proposed is more substantially specified by the function Rz·n, increases uniformly with an increase in τ and abrasive grain size. In this case the rate of increase using cast iron grit is much lower than with use of silicon carbide grit (Fig. 7).

Fig. 7.
figure 7

Dependence of actual surface area under spraying Rz·n on K: ∆) cast iron grit; ○) silicon carbide grit.

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It follows from Fig. 5 that there is a sharp reduction in n (by more than a factor of 1.5) with a certain increase in micro-projection height. With higher values of K there is some stabilization of Rz·n value, i.e., stabilization of the actual surface area for deposition.

This approach makes it possible to explain the extreme nature of the dependences shown in Fig. 8. An increase in adhesive strength proceeds as a result of an increase in actual specimen surface contact with a coating, and consequently on the number of individual bonds between them. With an increase in τ alongside an increase in specimen surface Rz the number of peaks n in a surface is reduced, and in this stage the value of the second function (n) prevails over the first (Rz) which leads to a reduction in actual surface area. Whence follows the reduction observed in coating adhesive strength with a base.

Fig. 8.
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Dependence of adhesive strength σad of coating with base on surface (St3) Rz: ●) alloy based on chromium; □) alloy based on nickel; ∆) alloy based on iron.

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During jet-abrasive treatment an abrasive stream is aimed normal to a surface being treated that gives the minimum component material removal [27] and makes it impossible to change component dimensions. With a change in treatment angle thinning increases so much that failure to take it into account leads to scrap. Therefore with small treatment angles it is necessary to make a tolerance for treatment.

A curve is shown in Fig. 9 for metal removal on τ with a an abrasive jet inclination angle to a specimen surface φ = 45°.With treatment for 40 sec there is a sharp increase in the intensity of metal removal that may be explained by occurrence of specimen surface work hardening and this leads to a reduction in toughness and a surface becomes more brittle and compliant for metal removal. Consequently treatment of a component unit area should not be conducted for more than 40 sec.

Fig. 9.
figure 9

Dependence of metal removal ∆P on τ with φ = 45° (a) and on treatment jet angle φ with τ = 60 sec (b ).

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Results of studying the effect of abrasive jet treatment clearly show (Fig. 9b ) that with angles of 35 and 45° there is a marked change in material removal. This makes it possible to recommend them during treatment at small angles. With a reduction in jet treatment angle the treatment spot is extended and acquires the shape of an ellipse. In this case treatment spot area increases.

A dependence is shown in Fig. 10 for the average amount of metal removal over the surface of a component treatment spot form a unit area ΔP/F on angle φ. Most critical from the point of view of metal removal is angle φ = 65° with which compared with φ = 90° metal removal increases by more than a factor of eight.

Fig. 10.
figure 10

Dependence of ∆P/F on angle φ.

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It should be noted that coating application should be performed immediately after surface treatment. In this case a break should be minimized and not exceed 2 h since a prolonged delay for an activated surface is accompanied by active oxidation reducing coating adhesive strength with a base.

For bases and different types of coatings it is necessary to select by experiment and to optimize a surface preparation method for surfacing. Moreover, in evaluating the effect of surface properties on coating adhesive strength consideration should be given to adhesion mechanisms.

CONCLUSION

These experimental studies make it possible to conclude that in order to prepare a component surface for plasma deposition a rational method is abrasive treatment with a heat-abrasive tool.

In order to obtain strongly adherent coatings and to restore worn components by plasma deposition the following surface abrasive treatment regimes are recommended: the distance of the jet gun nozzle to a treated surface of 35 – 40, treatment duration for a unit area of a component surface not more than 40 sec, and jet gun inclination angle with respect to component surface 40 – 45°.

In order to achieve optimum jet gun abrasive treatment parameters (Rz, L, η) for materials with hardness 170 – 230 HV it is necessary to perform treatment with abrasive particles up to 0.7 mm in diameter, and for materials with hardness of 350 – 450 HV with particles 0.7 – 1.2 mm in diameter.