Introduction

New structural materials and coatings are developed to offer better performance and reliability. New composi­tes extend the service life of products operating at high rates under elevated temperatures, mechanical loads or exposed to aggressive environments.

Borides of transition metals, such as TiB2 and HfB2 are promising ones since they have a high level of hardness, heat, wear and corrosion resistance [1 – 5]. There are few boride coating application technologies. Jayaraman S. et al. [6] produced HfB2 and Hf – B – N coatings by chemical vapor deposition (CVD). The hardness of the coatings is up to 40 GPa. The CVD technology is also used to apply ultra-high-temperature transition metal boride coatings onto porous substrates. The application technology for such coatings consist of thermal gas-phase decomposition of titanium, zirconium and hafnium borohydride solutions in high-boiling saturated hydrocarbons as borohydride and solvent vapor are passed through porous materials preheated to 250 °C, and then placed in a tubular reactor under vacuum (Dugin S.N. et al. [7]).

Composite HfB2/SiC coatings protect from oxidation in various aggressive environments [8 – 12] and are therefore of a great interest. Silicon carbide alloying with hafnium diboride significantly increases the structural strength at high temperatures, thermal conductivity and heat resistance while reducing the thermal expansion coefficient. Such coatings are extensively used under high temperatures and contact loads, e.g. in solid-propellant rocket engine components [13].

Spark plasma sintering (SPS) is used for sintering ceramic powders. This technology produces ultra-high-temperature and high-strength ceramics [14 – 16]. The standard temperature of HfB2 – SiC spark plasma sintering is 1800 – 2100 °C [17].

Despite their excellent performance and physical/structural properties, such coatings are not widely used due to their high brittleness and the lack of direct application technologies. Therefore, it is advisable to use boride composites with binding metals.

Gas cladding is an efficient technology for makin­g highly dispersed composite powders; one component is converted into a gas phase and then deposited on the surface of the other component. Iodine is used as a transport agent. Bogdanov S.P. [18; 19] studied the iodine transport of cladding components in detail [18, 19].

As components operated under high contact loads are restored a layer up to 1 mm thick [20] is mostly app­lied.

Thermal spraying is considered a suitable technology for making coatings of such thicknesses [21]. In particular, the microplasma technology can spray composite powders while maintaining their high physical and structural properties. 20 – 300 μm thick coatings are applied with a single pass.

The above-mentioned composite powders can be used as protective and repair coatings for mechanical parts exposed to high contact loads, variable temperatures and aggressive or corrosive environments in heat exchangers, steam generators, pipelines, valves and jet engines.

The purpose of this study is to apply the iodine transport technology to TiB2 /Ti and HfB2 /Ti composite powders, fine-tune the microplasma spraying process, and estimate the properties of the resulting coatings.

Materials and Methods

We used the following materials:

– PTOM-1 titanium powder, 10 – 100 μm grade from POLEMA, AO (see Table 1 for the chemical composition);

– titanium diboride powder, 1 – 4 μm grade (the che­mical composition is: 68.3 % titanium; 30.2 % boron; 0.1 % carbon; 0.05 % iron);

– HfB2 hafnium diboride powder, 3 – 12 μm grade, 99.8 % purity (the chemical composition is: boron 29 %; the rest is hafnium).

Fig. 1 shows the SEM images of the powders. The powders were mechanically mixed. 

The MeB2 :Ti mass equivalent ratio was 50:50 %, where Me is Ti or Hf. The mixtures were then synthesized by the iodine transport technology. The titanium was converted into a gas phase by a chemical reaction with iodine vapor and carried to the surface of ceramic particles. The mass transfer intensity depends on the temperature and the holding time. The temperature was 700 °C, and the holding time was 3 h. The resulting TiB2 /Ti and HfB2 /Ti composite powders, 20 to 80 μm grade were deposited with the microplasma technology on UGNP-7/2250 machine equipped with Kawasaki FS003N robot manipulator. The plasma generator power reached 2.8 kW and the arc current was 35 to 40 А at 40 V. Argon (2 l/min flow rate) served as the transport agent and plasma gas.

We studied the structure of the powders and the poli­shed cross-sections of the coatings on Tescan Vega 3 scanning electron microscope (SEM). PMT-3 microhardness tester was used for the coating microhardness testing by Vickers method.

The 2168 UMT abrasion testing machine performed accelerated wear resistance tests. It is a general-purpose machine with replaceable friction pairs and multiple avai­lable motions in a wide range of rpm and loads. Lubricants can be delivered to the friction area. The samples had coating sprayed on their ends. They were arranged as a ring-on-ring friction pair with continuous water cooling. The independent variables were the normal load applied to the ring, the ring rpm and the test time. All the samples were examined under the 0.5 MPa load at 100 rpm for 5 h.

Results and Discussion

We examined the structure of the TiB2 /Ti and HfB2 / Ti composite powders processed by the iodine transport technology. The typical structure is shown in Fig. 2.

The composite particles mostly inherit the shape of the original ceramic components, while some of the particles remain unclad and they are merged in agglomerates.

Fig. 3 and 4 show the SEM images of the TiB2 /Ti and HfB2 /Ti composite powder coating polished cross-sections respectively.

The images in Fig. 3 and 4 indicate that the coatings have clear interfaces with the substrate, without through pores. The dark and bright areas in the backscattered electron SEM images indicate that the titanium diboride and hafnium diboride particles are preserved in the coa­ting. Fig. 3 shows that the light areas are titanium and the dark areas are titanium diboride. In the SEM images of the coatings (Fig. 4) the darker areas are rich in titanium and the lighter areas are rich in hafnium, due to the higher atomic number of the latter. The titanium diboride coating changes its phase composition during spraying. The diboride is partially converted into titanium monoboride. A similar effect was given in [22; 23]. The hafnium diboride powder coating does not feature such transformations. Besides, the titanium dioxide phase is formed in the coatings due to the oxygen diffusion during spraying. It can be identified as the areas with an intermediate contrast between the areas containing the ceramic and the binder.

We examined the microhardness of the polished cross-sections of the coatings. The hardness of TiB2 (TiB)/Ti(TiO2 ) coating is 1320 HV under 100 g load and 1.0 rms deviation. The hardness of HfB2 /Ti(TiO2 ) is 1654 HV under 200 g load and 3.2 rms deviation.

The results indicate high hardness values of the ceramic components in the coatings. HfB2 /Ti has higher hardness because the ceramic component does not have a phase transition. As mentioned above, the titanium diboride is saturated with titanium at spraying, which resulted in monoboride formation. It slightly decreases the final hardness, since the hardness of the titanium diboride reaches 35 GPa, while that of the titanium monoboride is 28 GPa [24; 25] only.

We also tested the sprayed coatings for abrasion resistance. First, we used 41Cr4 steel as an abradant material (Table 2, tests 1 and 2). Then we tested the abrasion resistance of two samples with two types of coatings (Table 2, test 3). Table 2 shows the weight changes, weight wear and wear rates for each friction pair.

The weight wear of TiB2 (TiB)/Ti(TiO2 ) coating is 28 %, which is less than that of the steel part. The mass losses of the sample and the abradant materials were low. The similar test with a friction pair of a hafnium diboride coating sample and 41Cr4 abradant material showed that the sample mass loss increases while the steel body wear is 690 % higher than that of the coated sample. The reason is the higher hardness of HfB2 /Ti(TiO2 ) coatings.

Abrasion tests of TiB2 (TiB)/Ti(TiO2 ) and HfB2 /Ti(TiO2 ) coatings showed that the titanium diboride coating had higher wear resistance.

The weight wear of HfB2 /Ti(TiO2 ) coating is 290 % higher than that of TiB2 (TiB)/Ti(TiO2 ) coating despite its higher microhardness. This can be explained by better elasticity of titanium diboride coatings, since spraying forms titanium monoboride. The substance creates a chemical bond between crystal lattices of initial phases, which prevents the ceramic component removal by friction. The hafnium diboride coating lacks such a bond. As a result, HfB2 /Ti(TiO2 ) coating wear rate is higher than that of TiB2 (TiB)/Ti(TiO2 ) coating under identical contact loads.

Conclusions

We synthesized TiB2 /Ti and HfB2 /Ti metal-ceramic powders. The initial mass ratio of the components in the mixture was 50:50 %. The synthesis lasted for 3 h at 700 °C.

We studied the properties of TiB2 /Ti and HfB2 /Ti composite powder coatings applied with the microplasma technology.

The structural examination showed that the particles of ceramic components are fixed in the matrix without any through pores. During spraying the energy input changes the phase composition of titanium diboride composite powders from TiB2 /Ti to TiB2 (TiB)/Ti(TiO2 ).

Hardness of TiB2 (TiB)/Ti(TiO2 ) coating is 1320 HV and the hardness of HfB2 /Ti(TiO2 ) coating is 1654 HV. The abrasion tests indicated that for the steel grade 45H abradant material, the weight wear of the coatings is 28 % less than the wear of the steel body. TiB2 (TiB)/Ti coating offers the best performance with the lowest wear under a contact load with a steel body.