Introduction

Carbides of transition metals belong to the group of ceramic materials known as conventionally hard materials. This is effect of the character of chemical bound, crystallographic structure, and morphology. Nanocrystalline materials, playing the part of one of the phases of nanocomposite, can contribute to the occurrence of strengthening phenomena not existing in conventional materials of micrometric size.

Metal matrix composites (MMCs) are the focus of intense research and development worldwide for many industrial branches, where reduction in weight of elements along with the improvement in specific modulus, strength, thermal stability, wear resistance, and service life are required [1–4]. One of the ways for obtaining functional nanocomposite materials is the technology based on the selective laser melting of powder. This method belongs to promising rapid prototyping (RP) processes because of its ability to fabricate three-dimensional (3D) metal parts directly according to CAD model [5].

The study presents an experimental investigation about the oxidation process of the nc-TiC in 316L steel matrix nanocomposites prepared by selective laser sintering/melting (SLS/M) method using commercial 316L stainless steel powder and nanocrystalline titanium carbide powder as raw materials. The results of research on oxidation in dry air of nc-TiC/316L steel nanocomposites, reference 316L steel obtained by SLS/M method and of the used powders have been presented. The oxidation process was studied on the basis of TG-DSC-MS measurements.

Experimental

The original way of ceramic nanomaterials synthesis based on non-hydrolytic sol–gel method was elaborated [6, 7]. Using carbonized and purified nc-TiC particles the technology of production of nanocomposite structures based on the selective laser sintering/melting (SLS/M) technology was worked out. The MCP HEK Realizer II device was applied. The suitable gear and the modification of the parameters of the laser energy delivering allowed to use the device for work in the Partial Melting technology. Such configuration enables the research on obtainment of the nanocomposite material. As matrix the stainless steel (AISI 316L) and as the filler (nanoparticles) nc-TiC0,85 was used. Nanocomposite (TiC/steel) and reference (316L steel) samples for testing were formed by SLS/M technique in shape of balls of 3 mm diameter and of mass in the order of 160 mg. In the SLS/M process following parameters were used:

Ten vol.% of nc-TiC in steel matrix, the power of the laser equal to 5000 mA. Nanoparticles size, chemical and phase composition were determined with the following techniques: XRD (PANalytical PW3040/60 X’Pert Pro), TEM (JEOL JEM 1200EX).

The investigations on the oxidation process were carried out by thermoanalytical method using TG-DSC (SDT Q600, TA) coupled with MS (Thermostar GDS 301 Pfeiffer Vacuum) for gaseous products identification. Measurements were performed under non—isothermal (β = 10 K min−1) conditions, in temperature range from 298 to 1473 K. During the measurements temperature of the samples and TG, DTG, HF functions in time were registered. The mixed powder containing 10 vol.% of nc-TiC0,85 and 90 vol.% of 316L steel, the sintered 316L steel and the sintered nanocomposite containing 10 vol.% of nc-TiC0,85 in the 316L steel matrix were subjected to oxidation process.

Results

The average size of nc-TiC crystallites was in the order of 40 nm (Fig. 1). The powders mixed in ball mill before introducing them into the Realizer II device allowed obtaining of non-porous composites with uniform dispersion of nc-TiC particles embedded in the 316L steel matrix. X-Ray diffraction pattern of the nanocomposite is presented in Fig. 2.

Fig. 1
figure 1

a TEM micrograph and b electron diffraction image of purified nc-TiC powder

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Fig. 2
figure 2

X-Ray diffraction pattern of the composite containing 10 wt% of nc-TiC0,85 in steel matrix

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The composite material, containing nc-TiC0,85, obtained by SLS/M technique was characterized by satisfying dispersion of particles in steel matrix and increased hardness comparing to reference sample prepared from 316L steel powder [8].

In Fig. 3, the plots of TGu function in time and in Fig. 4 plots of DTG function in temperature of sintered samples composed of.

Fig. 3
figure 3

Comparison of the TGu curves for oxidation processes of the sintered 316L steel and the 316L steel/TiC (10 vol.%) nanocomposite, 10 K min−1 up to 1,473 K

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Fig. 4
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Comparison of the DTG curves for oxidation processes of the sintered 316L steel and the 316L steel/TiC (10 vol.%) nano-composite, 10 K min−1 up to 1,473 K

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10 vol.% nc-TiC particles in steel matrix and of the reference steel sample, both heated in synthetic air, are compared.

In Fig. 5, the DTG function and CO2 spectrum are presented, whereas in Fig. 6 TG and HF functions dependency on temperature for the oxidized sintered 316L steel are presented.

Fig. 5
figure 5

DTG function and CO2 mass spectrum in time. Oxidation of the sintered 316L steel in air, 10 K min−1 up to 1,473 K

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Fig. 6
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Dependency of TG and HF functions on temperature. Oxidation of the sintered 316L steel in air, 10 K min−1 up to 1,473 K

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Process of sintered 316L steel oxidation started at temperature of 1107 K. Simultaneously burning of structural carbon (CO2 evolution) took place. In oxidation process of 316 steel three stages have been distinguished. In the first stage mass growth was in the order of 0.135 wt%, in the second stage of 1,454%, and the third stage has not been finished.

Process of nanocomposite oxidation started at temperature of 936 K. In this process two stages have been distinguished. In temperature range of 930–1,200 K, the first stage finished and the second stage started.

In Fig. 7, the comparison of the diffraction patterns of steel and nanocomposite after oxidation at heating rate of 10 K min−1 up to 1,473 K are presented.

Fig. 7
figure 7

X-Ray diffraction pattern of the nanocomposite containing 10 vol.% of nc-TiC (a) and the 316L steel (b) after oxidation

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Among the oxidation products of both materials phases with structures corresponding to Fe2O3 and NixFe3-xO4 have been found. In case of sintered steel probably phases reacher in nickel, and in case of composite phases reacher in iron occured. Peaks from austenite and ferrite present in XRD spectrum give evidence that the products of nanocomposite oxidation form a thin layer. In the thin layer of products of nanocomposite oxidation some small fractions of phase rich in chromium, Cr2O3, have been additionally identified.

Analysis of phase composition of oxidation products confirmed observations made on the samples after oxidation process. The surface of nanocomposite was covered by thin passive layer with very good adherence to the composite substrate, containing besides iron oxides additionally chromium oxides, whereas in the thick, porous layer of scale, characterized by poor adhesion to 316L steel, hematite and magnetite rich in nickel dominated, what was confirmed by the results of the studies [9–13].

In Fig. 8,the dependency of TGu function on temperature of ball sample composed of 10 vol.% nc-TiC particles in steel matrix and the powder sample composed of 10 vol.% nc-TiC and 90 vol.% 316L steel heated at rate of 10 K min−1 up to 1,473 K in air have been presented. There was stated that the oxidation process of the powder starts at the temperature about 240 degrees lower in comparison to the sinter of the same composition.

Fig. 8
figure 8

Comparison of the dependency of TGu function on temperature for composites in form of sintered ball and powder. Oxidation of the 316L steel/TiC (10 wt%), 10 K min−1upto 1,473 K

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Analysis of phase composition of mixed powder, composed of 90 vol.% of 316L steel and nc-TiC (10 vol.%) heated in dry air at rate of 10 K min−1 up to 1,473 K, performed by XRD method (Fig. 9) showed the presence of the following oxidation products: Fe2O3, Ni x Fe3−x O4 and TiO2(rutile).

Fig. 9
figure 9

X-Ray diffraction pattern of the powder containing 10 wt.% of nc-TiC0.85 and 90 wt.% of 316L steel after oxidation

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Conclusions

The results of the oxidation processes of 316L steel and composite containing 10 vol.% of nc-TiC0,85 in 316L steel matrix sintered by SLS/M method and of the mixed powder have been compared.

It was stated that the nanocomposite material was characterized by better resistance to oxidation in air at high temperatures comparing to sintered 316L steel. After heating in dry air up to temperature of 1473 K at heating rate of 10 K min−1 on the surface of 316L steel/nc-TiC nanocomposite the thin passive layer was formed. In this layer, besides Fe2O3 and Ni x Fe3−x O4, Cr2O3 was identified by XRD method. Among the oxidation products of sintered steel two phases were identified: Fe2O3 and Ni x Fe3−x O4. Products of sintered steel oxidation formed thicker, loose layer of rust. Among the oxidation products of the mixed powder containing 10 vol.% of nc-TiC0,85 and 90 vol.% of 316L steel no Cr2O3 was found, whereas TiO2 (rutile) was identified. The process started at the temperature about 240 degrees lower in comparison to the sinter of the same composition.

The obtained results suggest that the nanocomposite oxidation proceeds according to different mechanism comparing to the mechanism of powder oxidation. The formed Cr2O3 is probably responsible for increased oxidation resistance of nanocomposite obtained by SLS/M method.