Properties of TiN-matrix coating deposited by reactive HVOF spraying.

Abstract TiN-matrix coating was prepared by reactive high velocity oxygen fuel (HVOF) spraying on carbon steel based on the self-propagating high temperature synthesis (SHS) technique in air. The phase composition, structures, and properties of TiN-matrix coating were analyzed using XRD, EDS, SEM and Vickers microhardness equipment. The anti-corrosion property in nearly neutral 3.5 wt% NaCl electrolytic solution was measured. The Weibull distribution of Vickers microhardness at different loads and their linear fitting were analyzed. The apparent fracture toughness of the coating was also calculated. The coating is composed of main phases (TiN, [TiN.sub.0.3]), minor phases ([Ti.sub.2][O.sub.3], Ti[O.sub.2]), and porosity. The anti-corrosion property of an HVOF-sprayed TiN-matrix coating in electrolytic solution is superior to that of AISI 316L stainless steel. The microhardness values from [1137HV0.sub.0.05] to [825HV.sub.1], are relatively high and have indentation size effect (ISE). With the increment of m, which increases with the increment of applied load, the microhardness values are more concentrated. The average value of apparent fracture toughness [K.sub.IC] is 4.62 MPa [m.sup.1/2]. It is higher than that of reactive plasma sprayed (RPS) TiN coating, which reflects the good toughness of a TiN-matrix `by reactive HVOF spraying.

Keywords Reactive high velocity oxygen fuel (HVOF) spraying, TiN-matrix coating, Vickers microhardness, Weibull distribution, Apparent fracture toughness, Polarization curve

Introduction

In recent years, surface coatings (especially ceramics) have been widely used in industries and aeronautic areas. (1-4) Due to no pollution and the excellent bonding strength property of in situ technology, the coatings deposited in situ have attracted the interest of researchers. (5), (6) Reactive thermal spraying is a good way to achieve coatings with good properties. (7) During spraying, hard ceramic phases (such as carbides, borides, and nitrides) are formed in situ by the reactions between the injected powders and the reactive gas.

Titanium nitride (TiN) coating has been extensively used as decorative coatings, wear-resistant coatings, diffusion barriers, and electrodes due to its superior wear resistance, erosion resistance, chemical inertness and thermal stability, heat resistance, low electrical resistance, and attractive golden color. (8) At present, TiN coatings are mainly obtained by physical vapor deposition (PVD) (9) or chemical vapor deposition (CVD). (10) However, their applications are restricted due to the low deposited efficiency and coatings that are always less than 10 [micro]m thick.

To obtain thicker coatings, a reactive plasma spraying (RPS) technique has been applied through reactions between titanium powders and nitrogen plasma jets. (11-15) Bacci et al. (11) prepared a TiN coating with a thickness of 60 [micro]m using RPS under the aid of a pure nitrogen atmosphere chamber at 500 bars. Kobayashi (12) used a gas tunnel-type plasma jet to prepare a TiN coating whose thickness was more than 200 [micro]m. Feng et al. (13) prepared a TiN coating that was 500 [micro]m thick using RPS in an environment containing nitrogen. In addition, Xiao obtained nanostructured TiN coatings that were more than 300 [micro]m thick by means of RPS in air. (16)

However, HVOF-sprayed coatings have superior adhesive strength and higher density compared to DC and RF plasma counterparts. (7) Furthermore, the TiN coating prepared by reactive HVOF spraying has not ever been reported before. To obtain good properties of a TiN coating, the research in this paper was completed. TiN-matrix coating was successfully achieved with reactive HVOF spraying, and the properties of the coating were discussed.

Experimental methods

Preparation of sample and the coating

Medium carbon steel (0.42-0.50 wt% C steel) with 40 mm diameter and thickness of 3 mm was used as the substrate material. Commercial Ti powders with particle size from 45 to 70 [micro]m (Jin Jiang Metallic Powder Co. Ltd. of Shanghai, P.R. China) were used as starting material. Nitrogen was selected as carrier gas. After finally polished with 400 mesh abrasive paper, the substrates were grit-blasted with alumina to get the fresh surface, ultrasonically cleaned in anhydrous ethanol, and then dried in cold air prior to the coating deposition. During the course of spraying, the Ti powders and the nitrogen reacted as follows:

2Ti + [N.sub.2] [right arrow] 2TiN (1)

A 300 [micro]m top coating of TiN-matrix coating was deposited without bond coat by a JP5000 HVOF spraying instrument. The spraying parameters are given in Table 1.

Table 1: HVOF spraying parameters

Spraying parameters      Value

Kerosene               0.31 L/min
Flow rate ([O.sub.2])  822 L/min
Flow rate (N.sub.2)    9.87 L/min
Power feed rate        70 g/min
Spraying distance      380 mm

Microstructure observation and property tests

The surface of the specimen was finely polished to avoid the effect of surface roughness before the X-ray diffractometer (XRD) measurement. The phase composition of the coating was analyzed by XRD (Bruker D8 Advance XRD) with Cu K[alpha] 1.5406 [Angstrom], and radiation at 40 kV and 20 mA. An energy dispersive M-ray spectroscope (EDS, EDAX, USA) was used to investigate the chemical composition of the coating. The cross section morphologies of the coating were studied with a scanning electron microscopy (SEM, JSM-6460, Japan). The microhardness of the TiN-matrix coating was measured by a Vickers tester (HX-1000, Shanghai, China) with load range from 0.05 to 1 kg and a dwell time of 40 s. The microhardness values were the average value of 10 measurement points in the middle of a cross section at each load. The cross section of the coating was polished before indentation, and the distance between two indentations was at least three times the diagonal to prevent stress field effects from nearby indentations. Weibull distribution, which is fit for the broad and dispersive distribution of microhardness for brittle ceramic material, was used to analyze microhardness values of TiN-matrix coating. The Weibull distribution of two parameters is given as follows (17):

F(x) = 1-exp [-([x/[eta]).sup.m]] (2)

where F(x) in equation (2) is the cumulative density probability function, x is the selected microhardness value, [eta] is characteristic value, and m is the Weibull modulus that reflects the dispersity of data in the distribution. The scale parameter [eta] gives 63.2% of the cumulative density. Equation (2) can be transformed as follows:

ln [-ln (1-F(x))] = m[ln (x)-ln (eta)] (3)

Therefore, a plot for ln [-ln (1-F(x))] versus In (x) will be a linear relation if the Weibull modulus is suitable. The function of F(x) = i/(n + 1) is supposed if the data is arranged in ascending order, where n is the total number of data points and i is the corresponding ordinal number. (17-20)

Calculation of apparent fracture toughness

The fracture toughness values of the TiN-Matrix coating at different loads were calculated based on equations (4) (13) and (5) (21).

[H.sub.v] = 0.463P/[a.sup.2] (4)

[K.sub.IC] = 0.016 [(E/H).sup.1/2] P/[C.sup.3/2] (5)

where a in equation (4) is half the length of the indentation diagonal, P is the applied load, E stands for Young's modulus, and c is the length of the radial crack measured from the center of indentation. The morphologies of indentations were investigated by SEM.

Electrochemical measurements

The potentiostatic polarization curves of the TiN-matrix coaling prepared by reactive HVOF spraying compared with AISI 316L stainless steel and bare carbon steel, which were studied using the CHI600C model instrument. A working electrode with an area of 1.0 [cm.sup.2] was used in the electrochemical test. The nonworking surface was covered with epoxide resin. A platinum pole and an Ag/AgCl electrode were used as counter and reference electrodes, respectively. Due to the good anticorrosion properties of TiN-matrix coatings, they have a potential application in seawater. To imitate the seawater environment, near-neutral 3.5 wt% NaCl solution (which is cheap and easily obtainable) was used as an electrolyte. It was static, naturally aerated, and at room temperature (20 [+or-] 5[degrees]C). (22), (23) Potentiostatic polarization tests were carried out from the initial potential of-1 V up to final potential of 3 V. Equipment with a scan rate of 0.05 V/s, a sample interval of 0.001 V, quiet time of 2 s, and sensitivity of 0.1 A/V was used in the experiment.

Results and discussion

Phase composition and structure

The phase composition of XRD analysis for the coating is shown in Fig. 1. From the XRD results, it is shown that the top coating is composed of TiN, Ti[N.sub.0.3], [Ti.sub.2][O.sub.3] and Ti[O.sub.2]. Five sharp TiN peaks appear in the spectrum, which suggests that the coating is mainly composed of a TiN phase. The contents of Ti[N.sub.0.3] and [Ti.sub.2][O.sub.3] are moderate, while the intensity of Ti[O.sub.2] is very low. That's the reason for naming the coating a TiN-matrix coating. Figure 2 displays morphologies of the cross section of TiN-matrix coating. The coating has a typical layer structure, with no distinct interfaces in the top coating and well bonding with the substrate, even without a bond coat. The thickness of the coating is about 350 [micro]m and the porosity of the coating is very low. From the consequences of the EDS analysis shown in Fig. 3 and Table 2, the bright phase (marked by Arrow 1) is clearly composed of Ti and N elements, while the dark phase (marked by Arrow 2) is composed of Ti, N, and O elements. With the XRD spectrum, it is clear that the bright phase consisted of titanium nitrides (TiN and Ti[N.sub.0.3]) and the other consisted of the mixture phase of titanium nitrides (TiN and Ti[N.sub.0.3])d titanium oxides ([Ti.sub.2][O.sub.3] and Ti[O.sub.2]).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Table 2: EDS analysis of a TiN-matrix coating

Element 1  Spectrum 1 (at.%)  Spectrum 2 (at.%)

NK               11.52              10.70
OK                 -                13.56
TiK              88.48              75.74

A higher velocity of combustion gases (more than 4-5 times the velocity of sound) from HVOF jets, rather than plasma spraying, can lead to the powder particles obtaining a very high kinetic and heat energy due to the heat transfer of the powders during spraying. Therefore, the melted particles can deform adequately and the TiN-matrix coating becomes denser, as well as less porous using HVOF spraying. Moreover, the efficiency of HVOF is high and more than 300 [micro]m in thickness can be achieved in a few minutes. Compared with the TiN coating prepared using RPS, (13) [[Ti.sub.2][O.sub.3]and Ti[O.sub.2] phases were formed due to the oxygen used for combustion and coatings being deposited in the atmosphere during spraying.

Vickers microhardness and indentation

Table 3 shows the mean, minimum, maximum, and standard error of Vickers microhardness for the cross section of TiN-matrix coatings. Furthermore, the relationship between microhardness and applied load is displayed in Fig. 4. It's very clear that the average Vickers microhardness increases from 825HV to 1137HV with the applied load decreasing from 1 to 0.05 kg. The Vickers microhardness is much higher than the TiN coatings prepared by RPS at the same applied load in former research (13), (24) due to the high kinetic energy of the powders, adequate deformation, and more dense coatings supplied by HVOF spraying.

[FIGURE 4 OMITTED]

Table 3: Vickers microhardness of the coating

Load (kg)  Mean  Min.  Max.  SE

1           825  730    970  23
0.5         947  870   1075  20
0.3         969  690   1410  59
0.2        1122  914   1584  67
0.1        1124  625   1533  82
0.05       1137  927   1486  54

Just as other researchers have already discovered, (25) the Vickers microhardness of TiN-matrix coatings also Table 2: EDS analysis of a TIN-matrix coatings also has indentation size effect (ISE), which is the measured microhardness decreasing with the increase of applied load. The possible reason for ISE is that the hardness equals the energy consumption for plastic deformation of unit volume. Because the surface area of solid materials will inevitably change during the course of indenting, the energy consumption of indentation is partially consumed for volume plastic deformation, and other energy consumption will transfer the indenting that remains on the cross section of the TiN-matrix coating. When the applied load increases gradually, the crack formation (which can release the high stress to reduce the energy) increases, and the energy provided by indentation consumes more and more. Accordingly, the recovery of indentation surface area decreases, so the phenomenon of ISE is manifested from the hardness value. (26)

From the diagram of the relationship between Vickers microhardness and applied load (shown in Fig. 4), it is clear that the slope between the applied load of 0.2 kg and 0.3 kg changes sharply because of an obvious variety of phase shapes in TiN-matrix coatings. This phenomenon can be explained by elastic recovery and the formation of microcracks. The phase in the coating will be crushed because of the adequate applied load. Moreover, the crushed phase will show as microcracks. When the applied load lowers (less than 0.3 kg), there is no formation of microcracks along the diagonal of indentation for inadequate pressure. For example, a little arc present in the brim of indentation (shown in Fig. 5) indicates good elastic recovery with an applied load of 0.05 kg. When the applied load is off, high stress around the indentation is changeable. More obvious ISE appears with low applied loads. Contrarily, the microcrack appearing along the diagonal or brim of indentation can release the high stress to reduce energy, as shown in Fig. 6. Less obvious ISE is shown due to the weakening of elastic recovery on surface area.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Weibull distribution of Vickers microhardness

The Weibull plot of the Vickers microhardness for the TiN-matrix coating on the cross section at different loads and linear fittings is shown in Fig. 7. The detail information supplied by the figures is summarized in Table 4. It is remarkable that all the Weibull modulus values are suitable for showing a satisfactory distribution. However, the Weibull modulus increases with the increment of applied load. The low modulus corresponds to a high variability in the microhardness measurement. Accordingly, the microhardness values are more concentrated with a high modulus. The hard phases may be responsible for this phenomenon. With a light load, the indentation size is short and randomness is enhanced. The values of microhardness will be much different when a light load acts on the hard phase and soft phase. Contrarily, any phase may be crushed with a heavy load so that the indentation sizes are more uniform. Therefore, the measured microhardness values show lower variability with a heavy load.

[FIGURE 7 OMITTED]

Table 4: Summary of the results obtained from the Weibull distribution
plot

Applied      Weibull   Error    Hardness      In (HV)
load (Kg)  modulus, m         range in HV

0.1           3.85     0.31     625-1533     6.44-7.33
0.3           4.91     0.89     690-1410     6.54-7.25
0.5           8.48     1.00     870-1075     6.77-6.98

Apparent fracture toughness

Fracture toughness is a key factor in estimating the resistance to crack generation and propagation. The damage of material due to mechanical action, thermal cycling, thermal shock and stress corrosion (27) can also be reflected by fracture toughness. The cross-sectional view of indentations at applied loads of 0.3 kg and 0.5 kg are exhibited at a magnification of x2000 in Figs. 6 and 8, respectively. Both of the cracks propagate along the extension of the diagonal. The parameters needed to calculate the apparent fracture toughness of the coating are listed in Table 5. To omit the titanium oxides in the coating, Young's modulus of TiN-matrix coatings is considered to be that of TiN coating--that is, 590 GPa. The average of apparent fracture toughness of the TiN-matrix coating at 0.3 and 0.5 kg load reaches 4.62 MPa. [m.sup.1/2], which is higher than the apparent fracture toughness prepared by RPS.(13)

[FIGURE 8 OMITTED]

Table 5: The parameters for calculating [K.sub.IC]

Load     a       HV     c       [K.sub.IC]
(kg)  ([mu]m)  (GPa)  ([mu]m)      (MPa.
                                [m.sub.1/2])

0.3   11.65    10.25  19.44     4.25 [+ or -]
                      [+ or -]  0.18
                      0.52

0.5   16.22    8.81   25.82     4.99 [+ or -]
                      [+ or -]  0.17
                      0.58

The superior properties of the reactive HVOF TiN-matrix coating should be owed to the unique structure and the toughening phenomenon. First of all, the structure of TiN-matrix coating is produced in situ by reactive HVOF spraying. The coating without pollution was deposited in situ and bonded layer by layer very well. High density and small pores hinder the formation and propagation of microcracks. Secondly, the oxides in the coating improve the fracture toughness. Although the existence of oxides in TiN-matrix coatings decreased the microhardness of the coating to some extent because of their reduced hardness, their interstratified structure in the main TiN phase is an important factor in slowing down the velocity of crack initiation. Last, but not least, the characteristic of HVOF spraying with a high velocity of combustion gases makes an important contribution to the denser coatings.

However, the calculated [K.sub.IC] value of the coating is an approximation. The accuracy of this method is not high due to multiple cracking, some pre-existent flaws, the residual stress in TiN-matrix coatings, and some error in measuring the length of crack using the SEM image. Moreover, the 0.016 coefficient determined by the shape of the indenter in equation (5) is an empirical value provided by Anstis et al. (21) The discrete coefficient of 0.016 is about 25%, making a relative error of approximately 30% of the [K.sub.IC] value. (26) On the other hand, the effects of the [K.sub.IC] value of [TiN.sub.0.3], [Ti.sub.2] [O.sub.3], and [TiO.sub.2] phases were neglected for the simple calculation of [K.sub.IC] value in TiN-matrix coatings. But it still can reflect the capability of the TiN-matrix coating in resisting crack generation and propagation, corrosion resistance, stress corrosion resistance, and so on.

Polarization curve

The anodic and cathodic polarization curves corresponding to bare carbon steel, AISI 316L stainless steel, and HVOF-sprayed TiN-matrix coating samples are shown in Fig. 9. Curve AR, KL, and JF in the graph are cathodic polarization curves of bare carbon steel, AISI 316L stainless steel, and HVOF-sprayed TiN-matrix coating, respectively. Contrarily, curve AB, LM, and FG belong to anodic polarization curves. The current increases with the increment of potential in both these regions. This result indicates that these regions arc active regions that correspond to the solution of surface material.

[FIGURE 9 OMITTED]

It is very obvious that a typical passive region (curve GH) is found for the HVOF-sprayed TiN-matrix coating. A protective film for substrates is formed and the current decreases with the increment of potential in the passive region. With the continuous enhancement of potential, the protective film of the TiN-matrix coating is disruptive at point H. Reversely, there is no passive behavior happening for the bare carbon steel and AISI 316L stainless steel in 3.5 wt% NaCl solution--it just has a transition zone (curve BC) between the active and passive regions for carbon steel. Then, bare carbon steel continues to dissolve (curve CD) and the current increases gradually. A protective film (curve DE) is formed at a relative high potential compared with TiN-matrix coatings. Austenitic stainless steels (in particular, AISI 316L steel) have been used in industrial applications due to their good corrosion resistance in different environments. (28) Compared with carbon steel, the corrosion potential of AISI 316K stainless steel increases and the corrosion current decreases. It shows that the corrosion resistance of 316L stainless steel is better than carbon steel. Similarly, the corrosion potential of the HVOF-sprayed TIN-matrix coating increases more sharply than AISI 316L stainless steel, and the corrosion current also decreases. It can be concluded that the coating can evidently improve the anticorrosion property of bare carbon steel. Dense coatings deposited by reactive HVOF spraying can hinder the corrosion of electrolytes very well.

Conclusions

The TiN-matrix coating of more than 300 [micro]m in thickness was deposited by successful reactive HVOF spraying in air. The typical layered structure of the coatings bonds well with each other. The coatings also bond with the substrate in a good condition even without a bond coat. Furthermore, the main characteristics of HVOF spraying were discussed above. The main conclusions are as follows:

(1) Higher average microhardness value were obtained using reactive HVOF spaying than RPS with the same applied load. The microhardness values increase from 825[HV.sub.1] to 1137[HV.sub.0.05] with obvious indentation size effect (ISE) in the coating. Satisfactory Weibull distribution is more concentrated with the increment of applied load.

(2) The reactive HVOF coating possesses high apparent fracture toughness about 4.62 MPa. [m.sup.1/2], which is relatively outstanding in ceramic coatings. The brim of the indentation has arc shape due to the good toughness. The excellent microhardness and apparent fracture toughness properties are determined by the characteristics of relative HVOF spraying.

(3) Thick coatings can remarkably improve the anticorrosion property of bare carbon steel. Compared with AISI 316L stainless steel, HVOF sprayed TiN-matrix coatings with a longer passive region and lower corrosion current possess an excellent anticorrosion property in 3.5 wt% NaCl electrolytic solution.

Acknowledgments The authors are grateful to Mr. W. Tang for the depositing of the coating and the XRD analysis by Mr. H. B Han in the instrumental analysis center of Shanghai Jiaotong University. This work was financially supported by the Opening Fund Program of China (No. KFJJ07-2). The authors are also thankful for the hard work of editors and reviewers on the paper.

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Z. Mao, J. Ma, J. Wang, B. Sun

State Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai 200240, People's Republic of China

e-mail: [e-mail]

Z. Mao

State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology. Beijing 100081, People's Republic of China

J. Ma

Material Institute, Hebei University of Science and Technology, Shijiazhuang 050054, People's Republic of China

[c] FSCT and OCCA 2008