High-temperature Short-term and Long Hardness of Sintered Compact and Porous Titanium-siliceous Carbide Ti3SiC2

The ternary compound of titanium-siliceous carbide Ti3SiC2, one of the representatives of nanolaminates, prepared by solid-phase sintering is investigated in compact and porous (θ=0.03-0.41) states. Features of its short-term and long-term hardness (Р=10 N) behavior in the temperature range from 20 to 1200 С at a holding time of 1-60 min were studied. It is shown that a temperature of about 700 С and holding time under load of about 10 min are critical values of the indentation procedure that correspond to an intensive decrease of hardness. The presence of porosity results in a decrease in hardness. А deformation scheme of compact and porous titanium-siliceous carbide Ti3SiC2 in the temperature range from 20 to 1200 С is proposed.


Introduction
Ternary titanium-siliceous carbide Ti 3 SiC 2 is a representative of a family of new materials -so-called nanolaminates [1][2][3][4][5][6][7][8][9][10][11][12][13][14].It is a material with a hexagonal crystalline lattice in which three close-packed layers of titanium atoms alternate with one layer of silicon atoms, and carbon atoms occupy all octahedral pores between titanium atoms.As a result the unit cell of the crystalline lattice has a layered structure with parameters -а=0.3066nm and с=1.767 nm.Specific features of this crystalline lattice are as follows.It has a very high value of the axis ratio с/а=5.76.Acting in it are three types of interatomic bonds -covalent, metal and ionic.Interatomic forces of silicon atoms Si-Si and those of silicon and titanium atoms Si-Ti are much less than the Ti-C forces [1,2].
M.W. Barsoum [1,2] showed that during loading of this material at room temperature edge dislocations are formed; they move and multiply in basal planes, concentrating in arrays.Their action on grain boundaries results in intergranular and intragranular laminar microdelamination, which is the controlling mechanism for relaxing arising internal stresses.Thus 4 mechanisms can act practically simultaneously to promote deformation: shear along basal planes of the grain; microdelamination -intergranular and intragranular; formation of shear and kink bands inside grains; microdelamination in shear and kink bands.In such conditions propagation of the formed microcracks is hampered.As a result titanium-siliceous carbide Ti 3 SiC 2 at room temperature loading possesses an ability to have residual deformation, which is actually pseudo-plastic [1,9,11].
Features of the Ti 3 SiC 2 crystalline lattice and peculiarities of its dislocation structure at the macrolevel result in a unique combination of physical and mechanical properties, which enables consideration of this material as a ceramics with unique properties.It has ductility, heat resistance, high temperature strength, thermodynamic stability up to decomposition temperature Т dec =2300 о С (at this temperature it decomposes by a peritectic reaction with formation of Si and TiC).
In spite of high brittleness at low and intermediate temperatures titanium-siliceous carbide Ti 3 SiC 2 can be a perspective material for use in products where a combination of technological effectiveness at room temperature (it's machinability is similar to that of graphite) with high values of specific both short-term and long-term high-temperature strength as well as fire resistance are required.
Practically all known papers are devoted to investigations of this material in a compact state.However, it would be a tempting possibility to employ it in a porous state.As a counterbalance of the inevitable decrease of strength characteristics one can expect an increase of its specific mechanical properties and economic efficiency of its application.
An earlier analysis of dynamic strengthening and weakening processes of sintered porous titanium-siliceous carbide Ti 3 SiC 2 at high-temperature deformation under uniaxial compression enabled obtaining qualitative results about the evolution of its dislocation structure and deformation process [10][11][12][13][14]. Results of the present research obtained by measurement of short-term and long-term hardness give some additional information related to the above work.
In [15][16][17] the structural changes taking place in hot-pressed compact Ti 3 SiC 2 during indentation at room temperature were investigated in detail.
The purpose of the present work is to establish regularities and features of deformation of sintered titanium-siliceous carbide Ti 3 SiC 2 in compact and porous states under indentation in a temperature range from 20 to 1200 о С (0.11-0.57Т dec ).

Experimental
The samples used for the research were fabricated earlier [10,11] by a technique which includes production of a material with required porosity by two-phase sintering of charge mixes of powders TiH 2 -TiC-SiC (size of particles is 3-5 µm) in vacuum at temperatures of 900 and 1200 о С (or 1400 о С) [10,13].An opportunity to use titanium hydride TiH 2 and its advantages for synthesis of the ternary compound have been shown earlier in producing binary intermetallic compounds [18].The phase composition of the material produced by such a procedure was controlled by X-ray phase analysis.The composition corresponded to stochiometric Ti 3 SiC 2 in volume of 0.9-0.95 with impurities of TiC, TiSi 2 and SiC.Values of porosity to be produced by this method are in a range of θ=0−0.5 and were determined by hydrostatic weighing.Cylindrical samples 10 mm in diameter and in height were fabricated.
The typical structure of porous titanium-siliceous carbide Ti 3 SiC 2 is shown in Fig. 1 [13].
Plate-shaped particles (grains) of the compound have the thickness of 1-2 µm and the cross-section size of 5-10 µm.Being curved and branched, with ledges and deepenings, the plates have different thicknesses.Adjoining between themselves during growth, such flat particles make contacts of a different degree of strength, i.e. from insignificant contact up to mutual intergrowth with the formation of bridges (isthmuses).As a result the porous material obtains a original frame structure without a primary direction in an arrangement of "ribs", their role being played by the plate-shaped particles [13].
In order to measure short-term (t=1 min) and long-term (t=1-60 min) hardness Vickers indentation was introduced with a load of 10 N.
The indentation was made in vacuum of 10 -3 Pa in a temperature range from 20 to 1200 о С.The length of indent diagonals obtained was in a range of 90-300 µm.The influence of temperature and porosity of the material on its short-term and long hardness was studied.

Dependence of short-term hardness on temperature
Indentation of titanium-siliceous carbide Ti 3 SiC 2 with various porosity in a temperature range of 20-1200 о С reveals some features of its behavior (Fig. 2).
Hardness of the compact material in the temperature range of 20-600 о С (0.11-0.34 Т dec ) is practically constant (Fig. 2а, curve 1).At temperatures above 600 о С (0.34 Т dec ) a sharp drop of hardness occurs.The rate of this drop is practically constant up to the temperature of 1000 о С (0.49 Т dec ).At higher temperatures the rate of the hardness drop increases.As one would expect, firstly, in the porous material with low («closed») porosity (θ=0.03-0.08) the level of the hardness values is decreased (Fig. 2a, curves 2, 3, Fig. 2b).Secondly, the temperature of the sharp drop of hardness decreases (for example, at θ=0.08 it S. A. Firstov 2а, curve 3).At 700-1000 о С repeated "stabilization" of hardness values occurs.At T≥1000 о С (0.49 Т dec ) the hardness values of the low porosity material lie near those of the compact material, at 1200 о С (0.57 Т dec ) these values are lower and become almost the same (Fig. 2b).
The increase in material porosity up to θ=0.28 ("open" porosity) leads, firstly, to a significant decrease of the hardness level (Fig. 2а, curve 4).Secondly, it leads to absence of the sharp drop and existence of an insignificant temperature dependence of hardness in the range of 20-800 о С (the hardness value decreases from 1230 MPa at 20 о С to 520 MPa at 1200 о С).
In the material with a porosity of θ=0.41 (Fig. 2а, curve 5) the temperature dependence of hardness is practically absent in a wide temperature range of 20-900 о С.The values of hardness are small (HV=500 MPa) and slowly reduced at temperatures above 900 о С (the decrease is up to HV=180 MPa at Т=1200 о С).

Dependence of hardness on the holding time
The process of hardness decrease of titanium-siliceous carbide Ti 3 SiC 2 was studied in the temperature range of 800-1200 о С (0.42-0.57Т dec ) for the values of holding time under load of 1, 5, 10, 20, 30 and 60 min.The obtained experimental data have shown (Fig. 3) that for all values of the holding time the increase in porosity gives rise to decreasing of both the hardness level and the rate of its drop.Increasing the temperature results in a decrease of the distinction, which becomes insignificant at 1200 о С (0.57 Т dec ).
The presented data demonstrate the dynamics of hardness decrease with an increase of the hold time under load that allows the effect of hardness decreasing of this material to be analyzed depending on both temperature and porosity.
The data presented in Fig. 3 were analyzed using logarithmic coordinates ln HV -ln t (Fig. 4).It was shown that the major part of the dependences obtained consists of two straight lines, with the boundary between them close to t=10 min.In addition it is established that it is difficult to define a unique description for all dependences in a time interval of t=1-10 min; at the same time in a time interval of t=10-60 min these can be described with the biggest precision by a known relationship of Н = a⋅ t -m , where a is the constant, m is the parameter whose absolute value characterizes the intensity of hardness decrease depending on time [19].
The obtained absolute values of m enabled definition of a temperature dependence for each value of porosity, i.e. quantitative expression of the effect of the hardness decrease of this material depending on time (Fig. 5).It is visible that the influence of porosity on the hardness decrease is an ambiguous one.This is due to the fact that in the material under study the effect of the hardness decrease is determined by several factors, which contribute depending on the structural state and deformation temperature.
On the one hand all processes controlled by dislocation mechanisms (plastic deformation, strengthening and weakening) develop in a compact part of the material.It was noted above that an effective mechanism of stress relaxation, microdelamination, acts during loading of this material simultaneosly with the formation and movement of edge dislocations.Intergranular microfracture also occurs under indentation [16,17].
On the other hand the presence of pores in the material provides an opportunity for the appearance of some dislocations on the pore surface reducing in this way the level of internal stresses.This mechanism is an alternative to microdelamination.The importance of contributions of the various hardness decrease mechanisms, which operate in this material (dislocation movement, formation of various dislocation structures, running of dislocations to the pore surface, microdelamination, intergranular fracture) should obviously be, first of all, dependent on the amount of pores and indentation temperature (Fig. 5).
In the case of significant "open" porosity its increasing reduces the m value, i.e. the hardness decrease effect (Fig. 5, curves 3, 4).This may be explained by a decrease of material mass under an indenter and, hence, by a decrease of the relative magnitude of stresses arising under it.Therefore, dislocation mechanisms of deformation prevail here as they are capable of providing a significant softening effect.
In the material with a small «closed» porosity (Fig. 5, curve 2) at 900-1050 о С (0.46-0.51 Т dec ) the hardness decrease effect is much lower than that in a highly porous material and is comparable with that in the compact material.This fact may be supposed to be in favour of the argument that microdelamination is the prevailing mechanism of hardness decrease in low porous and compact materials.It gives a smaller effect of hardness decrease than dislocation mechanisms.At temperatures above 1050 о С (0.51 T dec ) the effect of hardness decrease in the compact material abruptly increases.It may be related to the appearance of some more effective deformation mechanisms.At temperatures above 1150 о С (0.55 T dec ) switching on of alternative deformation mechanisms occurs (Fig. 5).

The Deformation Scheme of Compact and Porous Titanium-Siliceous Carbide Ti 3 SiC 2 in a Temperature Range of 20-1300 о С
Recently, M.W. Barsoum [1,2] proposed a model to describe the deformation behavior of titanium-siliceous carbide Ti 3 SiC 2 in a compact state at room temperature.Based on the above experimental data a question arises about utilization of this model as applied to porous materials at high temperatures.
The presence of bridges (isthmuses) in highly porous titanium-siliceous carbide Ti 3 SiC 2 is its essential structural feature determining the peculiarities of its mechanical behavior from the initial stages of deformation to fracture.In this case deformation of the material concentrates first of all in the isthmuses and mainly in those, which have the smallest cross section area.During loading moving dislocations have an opportunity to run on the free surface of pores, this way excluding increase of the enhanced stress concentration on grain boundaries and their subsequent microdelamination.Hence, in these sites a necessity for stress relaxation by a power-intensive process of grain delamination disappears.Due to this fact, the highly porous material gets an additional supply of plasticity (more precisely, pseudo-plasticity).Increasing porosity will result in the growth of this favourable effect (i.e., raise of the ability of residual deformation).It is necessary to remember however that strength characteristics of this material will be simultaneously reduced.
The proposed scheme of deformation of sintered compact and porous titaniumsiliceous carbide Ti 3 SiC 2 takes into account the following factors.i) Features of the crystalline lattice structure: lamination in an arrangement of titanium and silicon atoms and small interatomic forces acting between them [1,2].ii) Features of its dislocation structure: the availability of only edge dislocations during loading, the opportunity of their movement only in basal planes, a climb in the parallel planes at Т>700 о С (0.38 Т dec ) [13].iii) Morphology of grains (particles) of the porous material: presence of plates with the thickness of 1-2 µm and cross-section size of 10-15 µm interconnected by bridges (isthmuses).
As already mentioned, plastic deformation in this material is controlled by sliding of edge dislocations in basal planes with formation of planar arrays.Movement of edge dislocations in the parallel slip planes under the influence of the thermal factor (so-called climb), is known [20] to occur at temperatures that ensure diffusion of atoms of a crystalline lattice, i.e. as a result of plastic deformation due to diffusion mechanisms.
If the loading temperature is low enough, i.e. diffusion processes in the material cannot intensively develop, then in the compact state intergranular and intragranular microdelamination are intensified; in the porous state isthmuses (bridges) fail after some small value of deformation.In both states such microfractures can lead to the formation of microcracks of a critical size, causing subsequent fracture of the material.
The results of hardness measurements (Fig. 2) lead one to believe that diffusion processes in this material intensively develop at temperatures above 700 о С (0.38 Т dec ), i.e. edge dislocations get an opportunity to pass into the next planes by climbing.
In metals this usually occurs at temperatures of about 0.4 T melting [20].In this material the temperature of compound decomposition Т dec is analogous to T melting for metals: at Т dec =2300 о С titanium-siliceous carbide Ti 3 SiC 2 decomposes by a peritectic reaction with the formation of titanium carbide and silicon [1,2].Accordingly -0.4 Т dec ≈750 о С.As a possible reason of this correspondence the presence of a metallic component of interatomic force in the crystalline lattice of this compound can be pointed out.
In the temperature range of 800-1100 о С (0.42-0.53 Т dec ) the process of edge dislocations climbing in the parallel planes is intensified, which is revealed by growth of the hardness drop rate (Fig. 2).At Т>1100 о С (Т dec =0.53) the hardness decrease effect reaches a significant value (Fig. 2) being especially essential at an increase of the holding time under load (Fig. 3-5).
Confirming a qualitative change of the material state possessing enhanced deformability at high temperatures is also the fact that inside hardness indentations made at 1200 о С (Т dec =0.57) shear bands are observed; whereas these are absent for indentation at T≤1000 о С (0.49 Т dec ) [14].

Conclusions
1. Characteristic features of the temperature dependence of short-term hardness of the compact and low porous titanium-siliceous carbide Ti 3 SiC 2 are as follows: abrupt hardness drop at temperatures above 600 о С (0.34 Т dec ), non-monotonous hardness decreasing with the subsequent temperature increase and low values of hardness at high temperatures.The hardness of highly porous materials monotonously decreases with increasing temperature.
2. The dependence of hardness H on the holding time t of titanium-siliceous carbide Ti 3 SiC 2 is a power function H = a ⋅ t -m at t=10-60 min.The absolute value of m has physical meaning as a parameter of hardness decrease in time under action of constant load P=10 N.
3. It was established that in this material the influence of porosity on short-term and long-term hardness is different depending on the porosity and indentation temperature.
The low porous material (within the range of closed porosity) exhibits behavior similar to a compact material (effect of abrupt drop) for short-term indentation in a temperature range of 20-1200 о С (0.11-0.57Т dec ); the increase of porosity results in a decrease of hardness.For long-term indentation the behavior of the material is similar to a compact material in the temperature range of 900-1050 о С (0.46-0.51 Т dec ) (equally low value of hardness decrease parameter m).