Elsevier

Ceramics International

Volume 42, Issue 12, September 2016, Pages 13378-13386
Ceramics International

Influence of loading rate on nanohardness of sapphire

https://doi.org/10.1016/j.ceramint.2016.05.091Get rights and content

Abstract

This work reports the loading rate effect on nanohardness of sapphire. The intrinsic nanoscale contact deformation resistance of sapphire increased with the loading rates following empirical power law dependence with a positive exponent. The results showed a significant enhancement (e.g., ~66%) of the nanohardness of sapphire with the increase in loading rates from 10 to 10,000 μN s−1. These results were explained mainly in terms of the maximum shear stress generated underneath the nanoindenter, dislocation density and critical resolved shear stress of the sapphire.

Introduction

Sapphire is a leading candidate material for many extreme environments e.g., ground and air vehicle windows [1], optical lenses exposed to harsh environments [2] and transparent armors [3], [4]. All such applications involve contact with air borne hard dust particles and moving projectiles e.g., bullets, splinters etc. from regular weapons as well as improvised explosive devices. It is true that alumina is one of the most well known armour materials that has been used as a protective material against regular weapons. But in view of global insurgency and consequent attacks made by improvised explosive devices on public lives and society at large it is important to understand the deformation behaviour of alumina at microstructural length scale; to make them more usable for protective applications which were not thought of in earlier times when it was used only to protect against regular weapons. Indentation technique offers one unique way to study the contact induced deformation and controlled fracture events over a large length scale. Particularly in this context the still evolving nanoindentation technique [5], [6] comes handy as it can help us to study the micro- and nano-scale evolution of contact induced deformation and/or damage initiation over a length scale that can be as small as the microstructural length scale itself e.g., sub-μm to a μm to as big as a length scale that covers a number of microstructural length scale units, e.g., few μm to tens of μm. Therefore, it is not at all surprising to note that huge amount of literature exists [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22] on the nanoindentation behaviour of sapphire. These reports typically encompassed dedicated experimental work [5], [7], [8], [9], [10], [11], [12], [15], [18], [19], [20], [21], [22], theoretical work [9], [13], [14], [16], [17] and few attempts which combined both experimental investigation and theoretical interpretation [19], [20], [21].

However, dedicated study on the effect of loading rate (Ṗ=dPdt) on nanoindentation behaviour of sapphire was far from significant [19], [20], [21]. For instance, the magnitudes of (Ṗ) were varied in the range of as small as 35 μN s−1 to as large as 0.4 mN s−1 [19], [20], [21]. One interesting characteristic feature of these investigations [5], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22] was the presence of “pop-in”s that signify the transition from classically elastic to elasto-plastic deformation of sapphire under nanoindentation. It is true that particularly in ceramic thin films and especially in ceramic coatings pop-ins can occur due to crack generation beneath the nanoindentation and not due to plasticity. But, in the case of structural bulk ceramics like the polycrystalline alumina ceramics and the single crystal alumina of the present work, the corresponding loads at which pop-ins were reported were of the order of mN and μN [5], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22] and hence, such a small load is highly unlikely to cause crack generation beneath the nanoindentations in them as they require much higher loads for crack initiation [3], [4]. However, a definite unambiguous mechanism of the genesis of pop-ins is yet to be unequivocally established. The ultralow load at which they initiate is called the critical load (Pc) corresponding to a critical depth of penetration (hc) [5].

Based on pertinent literature data [5], [7], [8], [10], [11], [16], [18], [19], [20], [21] of alumina single crystals, a typical illustrative scenario for nanoindentation load (P), loading rate (dP/dt), critical load (Pc), nanohardness (H) and Young's modulus (E) are presented in Table 1. The data from literature survey (Table 1) showed that there was wide variation in the values of Pc. For instance, it was reported [5] that Pc of sapphire was as high as e.g., ~10 mN [5]. On the other hand as reflected in data of Table 1, other researchers reported that Pc of sapphire was low as e.g., 420 μN [19], [20]. It was interesting to note that the loading rates were not always reported in literature data [5], [7], [8], [10], [11], [12], [13], [14], [15], [16]. It could have been possible to calculate the loading rates from the experimental details provided the loading times were made available in the aforesaid references. However, in absence of the loading times, the loading rates could not be exactly calculated for the works reported in [5], [7], [8], [10], [11], [16], as reflected in literature survey provided in Table 1.

The magnitudes of Pc were reported to be dependent on the choice of crystallographic orientations of the sapphire crystal e.g., C-, M- or R-plane [5], [19], [20], [21]. But an equivocal picture was yet to emerge whether Pc was dependent on loading rate (Ṗ) or independent of it [19], [20], [21]. For instance, work reported in [19], [20] suggested that the magnitudes of Pc were sensitive to variations in (Ṗ). On the contrary, work reported in [21] indicated that the magnitudes of Pc were insensitive to variations in (Ṗ). It also appeared that it was a complex function of the particular plane concerned and the range of loading rates. For instance, with increase in (Ṗ) from 35 to 400 μN s−1 on the (0001) plane e.g., the C-plane of sapphire the magnitudes of Pc gradually decreased from ~620 to 520 μN [19], [20], [21]. But, as far as the (101̅2) plane of sapphire was concerned Pc did not show any kind of systematic variation with variations in Ṗ [21]. On the contrary, when loading rate (Ṗ) was enhanced from 20 μN s−1 to 100 μN s−1 the magnitudes of Pc had increased from ~380 to 540 μN [21] on the (1012) plane of sapphire. For the same plane, with further increase in loading rate (Ṗ) from 100 μN s−1 to 200 μN s−1 the magnitudes of Pc had decreased from ~540 to 360 μN [21]. However, the magnitudes of Pc had again regained back e.g., from ~360 to 540 μN when an even higher loading rate e.g., 100 μN s−1 to 200 μN s−1 was used during nanoindentation on the same plane [21]. These results clearly pointed out that the nature of variations of Pc with respect to variations in (Ṗ) was yet to be unambiguously established.

For a C-plane sapphire the magnitude of yield point load (e.g., Pc) varied in the range of ~700–850 μN for Ṗ varying between 100 and 1500 μN s−1 [22]. Nanoindentation conducted on the R-plane gave a comparatively lower Pc of ~400–500 μN [22]. These facts established that the present knowledgebase about loading rate dependence of critical load was far from comprehensive and sufficient. It acted as the motivational backdrop behind the present work.

Thus, the objective of the present work was to study the loading rate effect on nanohardness of sapphire. The loading rate was varied over a wide range e.g., 10–10,000 μN s−1. The other objectives were to examine if nanoscale plasticity events were really occurring in sapphire during nanoindentation. Further, it was decided to check out that if the nanoscale plasticity events happened in sapphire whether their occurrence was affected at all by the variations in loading rate. Finally, it was planned to examine the roles of maximum shear stress generated underneath the nanoindenter, dislocation density and critical resolved shear stress in influencing the physics of deformation in the case of the sapphire.

Section snippets

Materials and methods

The sapphire (0001) sample grown by the Verneiul process was obtained commercially from the Union Carbide Corporation. A nanoindentation machine (Tribo Indenter Ubi 700, Hysitron Inc., Minneapolis, MN) was used to evaluate the nanomechanical properties of sapphire. The depth sensing resolution of the machine was 0.04 nm. The machine was capable to resolve load variations of even as small as 1 nN. The machine provided a surface topography of constant contact force in scanning probe microscopy

Load-depth ((P-h)) plot analysis

The (P-h) plots obtained from the nanoindentation experiments are shown in Fig. 1(a). The most distinguishable feature observed from the (P-h) plots (Fig. 1(a)) was the clear and distinct presence of multiple serrations (the occurrence of a single serration is shown as inset of Fig. 1(a)). This is further confirmed from the higher magnification view of the (P-h) plots shown in Fig. 1(b). The presence of these multiple serrations implied the occurrences of multiple “micro-pop-in” and “

Discussions

The magnified views of the loading parts of the (P-h) plots clearly showed the presence of “multiple micro-pop-in” events in the present sapphire sample, Fig. 1. These “multiple micro-pop-in” events are the signatures of the nanoscale plasticity events occurring inside the sapphire under nanoindentation. Similar phenomena were also reported for sapphire [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], polycrystalline alumina [24], [25],

Conclusions

The present work reports the first ever study on the effect of variations in (Ṗ) from e.g., 10 to 10,000 μN s−1 on the nanohardness of sapphire. The nanoscale contact deformation resistance i.e., (Pc) increased with (Ṗ). This unique observation suggested an interesting capability of sapphire to resist externally applied nanoindentation load, P. In fact the resistance had saturated out at higher loading rates because Pc showed empirical power law dependence on (Ṗ) with a positive exponent. The τ

Acknowledgements

The authors (MB and AKM) are grateful to the Director, CSIR-Central Glass and Ceramic Research Institute (CGCRI), Kolkata for his kind permission to publish this paper. These authors appreciate the infrastructural supports received from all colleagues and particularly those received from the colleagues of the Advanced Mechanical and Material Characterization Division (AMMCD). Finally, the author MB gratefully acknowledges the financial support received from CSIR (Ack. no.: 163216/2k13/1).

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