Original Article
Nanoindentation and tribology of ZrB2 based luminescent ceramics

https://doi.org/10.1016/j.jeurceramsoc.2020.03.021Get rights and content

Abstract

Nano-mechanical and wear characteristics of ZrB2 + Al2O3:Cr3+,Nd3+ (Al2O3 co-doped with Cr3+ and Nd3+ ions) luminescent ceramic composites have been investigated using nanoindentation and tribological tests. Nanoindentation was carried out using a Berkovich diamond tip applying continuous stiffness measuring (CSM) mode with a maximum depth of 200 nm. The wear behaviour was studied during dry sliding in the air with a SiC counter body at applied loads of 5 and 50 N, and sliding speed of 0.1 m/s. The nanohardness of the ZrB2 and Al2O3 are very similar with mean values of 34.2 GPa and 32.6 GPa, respectively The Young modulus for ZrB2 is higher with a mean value of 555.6 GPa in comparison to the Young modulus of alumina with mean value of 473.8 GPa. Pop – in effect was found during the indentation of ZrB2 grains at indentation loads from 0.2–8.0 mN. The lowest coefficient of friction was measured for ZrB2 + 8%Al2O3:Cr3+,Nd3+ at 5 N load with value of approximately 0.5. The wear rate of the system ZrB2 + 32 % Al2O3:Cr3+,Nd3+ at 5 N is significantly lower in comparison to the wear rate of ZrB2 + 8% Al2O3:Cr3+,Nd3+ but at a load of 50 N the wear of the systems is very similar approximately 1. 10−5 mm3/N.m. Tribolayer formation connected with debris origin, oxidation and tribochemical reactions were characteristic for both composites with similar chemical composition but different size and thickness of tribolayers.

Introduction

Zirconium diboride (ZrB2) is a transition metal boride, characterised by unique combinations of mechanical and physical properties, including high melting points (>3000 ◦C), high thermal and electrical conductivities, chemical inertness against molten metals, and great thermal shock resistance, [[1], [2], [3], [4]]. The densification of ZrB2 without additives is difficult due to its low intrinsic sinterability, caused by the strong covalent bonding of ZrB2 and low volume and grain boundary diffusivities. This is one of the reasons why, over the last decade, different ZrB2 based particulate composites with SiC, B4C, MoSi2, etc., have been developed with enhanced sinterability, mechanical properties and oxidation resistance [[5], [6], [7], [8], [9], [10]]. These composites have been processed using various sintering methods including hot pressing (HP), spark plasma sintering (SPS), reactive hot pressing (RHP) and pressureless sintering (PS).

There are several papers dealing with the characterization of the wear of ZrB2 based ceramics using different tribological methods and conditions [[11], [12], [13], [14], [15], [16], [17]]. K. Umeda et al. [11], investigated the tribochemical response of hot pressed ZrB2, ZrB2+B4C and ZrB2+B4C + SiC ceramics in air and in de-ionized water using a reciprocating pin-on-block test. The coefficients of friction were about 0.95 for all systems at a low relative humidity (< 20 %), but they decreased with an increasing relative humidity. Chakraborty et al. [12] investigated the influence of direct current pulse on–off patterns on densification, and tribological properties of the spark plasma sintering of ZrB2 using a scratch test. It was found that the maximum relative density (98.65 %) is achieved at 50 ms pulse on (ton) and 5 ms pulse off (toff) time conditions, and that the wear volume and wear rate at a 10 N load was 4.05 × 104 μm3 and 1.01 × 10−3 mm3/(N·m), respectively.

Sonber et al., [13] investigated the friction and wear properties of hot pressed zirconium diboride at different loads and frequencies under reciprocative sliding, using the counter body of a WC-Co ball. According to the results the coefficient of friction for monolithic ZrB2 was between 0.8 and 0.4 at different load and frequency. The increase in load from 5 N to 20 N decreased the COF from 0.84 to 0.52 at 10 Hz frequency. The specific wear rate of zirconium diboride was approximatelly 10−6 mm3/N·m. Both abrasive and tribo chemical reaction wear mechanisms have been observed with abrasive grooves, tribochemical layer and wear debris formation with oxygen in the wear scar.

The scratch resistance and wear parameters of ZrB2+SiC composites, prepared by hot pressing with different sources of SiC, were investigated by D. Debnath et al. [14]. It was found that the interconnected network, and better contiguity between grains of ZrB2+SiC composites and the impurity content of the starting powders, can play significant roles in achieving high tribological properties of the composites.

Recently Medved et al. [15] investigated the wear characteristics of ZrB2 based composites under dry sliding conditions in air at room temperature and described the dominant wear mechanisms. Abrasive wear behavior of ZrB2 - 20 vol.% MoSi2 composite has been studied by T.R. Paul et al., [16] using a different tribologal testing method in the form of electroplated diamond disk as the counter body. According to the results, the applied load and sliding velocity plays an important role on specific wear rate and surface roughness of worn surfaces. The dominant mode of wear was identified as abrasion, grain pullout and micro cracking control the prevailing wear mechanism.

Nanomechanical characterization of ZrB2 based ceramics was carried out during the last decade by different reseachers, [[17], [18], [19], [20], [21]].

Guicuardi et al. [17] performed low-load nanoindentation tests on a polycrystalline ZrB2-based ceramic and reported that pop-in loads and pop-in extents were statistically distributed with a mutual strong correlation. According to the results the critical shear stresses at pop-in were in good agreement with the theoretical shear strength of ZrB2. The experimental pop-in extents were also compared to a simplified model developed for homogeneous dislocation nucleation. T. Csanadi et al. [18] investigated the influence of grain orientation on nanohardness and indentation modulus of ZrB2 grains in polycrystalline zirconium diboride using instrumented indentation. Detectable plasticity occurred around the indents. The nanohardness decreased from the basal towards prismatic orientations with a minimumvalue corresponding to a rotation angle of ∼50–60°. The obtained indentation modulus showed reversed tendency with a maximum around a tilt angle of ∼70–80°. Detailed analysis of the measured indentation modulus in comparison with that calculated using the model of Vlassak and Nix based on the single crystal elastic constants of ZrB2 revealed a similar tendency.

Anisotropic elasto-plastic transition of ZrB2 grains in ZrB2 + 10 wt% SiC and ZrB2 + 10 wt% B4C composites was investigated during nanoindentation by Csanadi et al. [19,20]. The results proved that the nano hardness and indentation modulus of ZrB2 grains exhibit evident orientation dependence. The calculation revealed homogeneous dislocation nucleation with similar critical resolved shear stress for each slip system close to the theoretical value (∼35 GPa). Based on the analysis of the orientation dependent maximal resolved shear stress, anisotropic dislocation nucleation was inferred in ZrB2 grains for three different regions of Φ with single and simultaneous activation of the investigated slip systems. Mao et al. [21] used nanoindentation with Berkovich indenters for the study of nanoscale pop-in phenomena in polycrystalline α-Al2O3 and single crystal α-Al2O3 (0 0 0 1). According to the results, the different radii and loading rates have significant effects on the pop-ins formation, stress distributions and dislocation nucleations. The hardness show a strong ISE within the range of the indentation depth h <100 nm, i.e. the value decreases with increasing indentation depth. At high indentation depth the hardness for α-Al2O3(0 0 0 1) is about 27.5 ± 2 GPa and for polycrystalline Al2O3 is about 30 ± 3 GPa.

Recently Naughton-Duszova et al., similar to several researchers in the world during the last decade, developed luminescent ceramics and studied the influence of Al2O3:Cr3+,Nd3+ addition on microstructure development, physical, macro-mechanical and luminescent properties of ZrB2 + Al2O3:Cr3+,Nd3+ ceramic composites using Al2O3 dopped with 0,5% Cr and 1% Nd, [[22], [23], [24]]. They found that the addition of aluminium oxide Al2O3:Cr3+,Nd3+ significantly improved the sinterability of the ZrB2 + Al2O3:Cr3+,Nd3+ ceramic composites and had a positive effect on the mechanical properties of the composite as well. On the surface of components prepared based on such a luminescent ceramic system with self-temperature monitoring functionality, the heat distribution and overheating spots will be possible to recognize in real time with the aim to save the lifetime of the components.

The aim of the present contribution is to investigate the deformation, damage and wear characteristics of ZrB2 based luminescent ceramic composites using instrumented nanoindentation and tribological methods.

Section snippets

Experimental materials and methods

The experimental materials, the ZrB2 + Al2O3:Cr3+,Nd3+ composites with 8 and 32 wt.% of additives, were processed by Spark Plasma Sintering (SPS) in argon at temperatures 1550−1600 °C under 50 MPa pressure with a heating and cooling rate of 200 °C/min and soaking time of 10 min. (HPD5 - FC). Young’s modulus of the composites was determined by ultrasonic wave transition method measuring the velocity of ultrasonic sound waves passing through the material using an ultrasonic flaw detector

Results and discussion

According to the results the addition of Al2O3:Cr3+,Nd3+ has a positive effect on the mechanical properties of the composite, the macro-hardness increased from 14.5 to 16.2 GPa and the fracture toughness from 3.91 to 5.89 MPa m1/2 in comparison to the values of the monolithic system, Table 1, [22,25]. The increased hardness was explained with the increased content of alumina which resulted in incresed sinterability and relative density.The increased fracture toughness is connected with the

Conclusions

The aim of the present contribution was to investigate the deformation, damage and wear characteristics of ZrB2 based luminescent ceramic composites using instrumented nanoindentation and tribological methods under dry sliding conditions in the air at room temperature.

The main results are as follows:

  • the nanohardness of ZrB2 and Al2O3 grains is very similar with values of ; 34.2 ± 1.6 GPa and 32.6 ± 1.3 GPa, respectively;

  • the Young modulus for ZrB2 and Al2O3 grains are 555.6 ± 31 GPa and

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the financial support from projects: APVV-15-0469, APVV-14-0385, MNT-ERA Nicre, VEGA 2/0130/17. This work was realized within the framework of the project „Research Centre of Advanced Materials and Technologies for Recent and Future Applications „PROMATECH“, ITMS 26220220186, supported by the Operational Program “Research and Development” financed through the European Regional Development Fund. The authors acknowledge the support of the project FNP – No.

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