1 Introduction

Wear of mechanical tools occurs during cutting or drilling. Especially in petroleum or geothermal energy survey and exploitation, tool performance is the main factor affecting excavation or extraction costs. Rock abrasivity has a significant influence on bit wear (Karfakis and Heins 1986). Over the last few years, several novel drilling technologies have been developed to improve drilling efficiency (Timoshkin et al. 2003; Lehmann et al. 2015). Rossi et al. (2020a; 2020b) proposed a combined thermo-mechanical drilling technology which can enhance the drilling performance in hard rock formations. The bottom hole assembly is made by a modular bit design, which incorporates the flame jet nozzles and the drilling cutters. A flame temperature above 2000 °C can be reached during rock drilling, and the penetration depth operated by flame-assisted drilling is much greater than that by conventional rotary drilling. Hence, it can be imagined that high temperature may reduce rock hardness, strength and abrasivity, enlarge material removal, and in turn enhance drilling efficiency.

Abrasivity as an intrinsic property of rocks defines the extent of wear or loss when interacting with other materials. Determining rock abrasivity relies on assessing tool wear resistance in direct contact with rocks. This includes a wide range of measurements, from real- or large-scale cutting or drilling tests, via simplified mechanical model tests, to mineralogical-petrological analysis of constituent minerals (Plinninger 2015). The constituent minerals with respect to their composition, content and hardness have a significant influence on rock abrasivity. Among them, quartz with Mohs hardness number of 7 stands out as a mineral that exists in many rocks including igneous, sedimentary, and metamorphic rocks (e.g., granite, sandstone or gneiss). Consequently, it holds a position of dominance in affecting the rock abrasivity. The cutting or drilling tool steels have a Mohs hardness of approximately 5.5. Minerals exceeding this value are regarded as abrasive, but minerals like fluorite, apatite, orthoclase and feldspar, falling within the Mohs hardness range of 4 to 6, also have abrasive potential. Moreover, geological environment involving reservoir temperature and in-situ state of stress have a significant influence on rock properties like strength, porosity and density. Taking quartz sand, sandstone and quartzite as an example (Fig. 1): sand is the weathering product of sandstone, and quartzite is the metamorphic transformation of sandstone caused by high temperature and pressure. The quartz content of sand, sandstone and quartzite is more or less equal, but the unconfined compressive strength (UCS) of these three materials become greater when subjected to high temperature and pressure, while the porosity shows an opposite pattern. Literature reviews showed that rock abrasivity can be related to its strength properties, such as the compressive and tensile strengths (Er and Tuğrul 2016; Capik and Yilmaz 2017; Zhang et al. 2020a; Wang et al. 2021a). It is reasonable that geological conditions can also affect rock abrasivity.

Fig. 1
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Evolution of compressive strength and porosity with increasing temperature and pressure for quartz-related rock materials (Plinninger 2015, modified)

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Among the mechanical testing methods for assessing rock abrasivity, the Cerchar abrasivity test emerges as the most widely used approach, and recent advancements in test apparatus have expanded an ability to measure additional parameters during scratching, yielding new insights for estimating tool wear and evaluating drilling efficiency. A pioneering work was performed by Rossi et al. (2018). They tested sandstone and granite at temperatures of 25 °C and 800 °C, respectively. It was found that CAI values measured at 800 °C are lower than those at 25 °C. Moreover, the authors concluded that, compared to the conventional rotary drilling method, the combined thermo-mechanical drilling can strongly reduce tool wear and enhance drilling efficiency. Considering that CAI values vary under high temperature and/or pressure conditions, Eberl et al. (2008) studied the temperature effect on rock abrasivity, aiming to see whether the transition of quartz minerals affects the abrasivity. Results showed that CAI decreases as temperature reaches 900 °C for both granite and sandstone due to mineral melting and structure disintegration of the rock. Alber (2008) investigated the influence of confining pressure on rock abrasivity after conducting a set of Cerchar tests with Hoek’s cell. Their results revealed a noteworthy disparity in CAI when lateral confinement is applied. Confinement leads to higher CAI values. Ji et al. (2020) discussed the influence of temperature as well as confining pressure on rock abrasivity by using granite samples. Following results were obtained: (1) reservoir temperature has only a minor effect on CAI in comparison with treatment temperature, (2) confining pressure strongly affects CAI, and (3) tool wear increases with increasing depth. More recently, Abu Bakar et al. (2023) studied the combined effect of temperature and confining pressure on rock abrasivity. After heat treatment, two types of granite were tested under different lateral confinements using Hoek’s cell. Results showed that, for both granites, the CAI values decrease with increasing temperature from 25 to 500 °C at all confinement conditions, but at 600 °C, the CAI values become nearly equal to those at ambient temperature. A linear increase in CAI with increasing confining pressure is observed for all treatment temperatures.

The major aim of this research work is to identify the critical temperature beyond which a significant reduction of the abrasive potential occurs. The understanding how heating can affect rock abrasivity is essential to develop new strategies and technologies to reduce tool wear and to enhance drilling efficiency. The Cerchar scratch test can be considered as an unlubricated tool combining indenting and scratching or cutting. Two-body abrasion (hard stylus scratches soft rock, as well as abrasive minerals scratch hard stylus) occurs predominantly, but three-body abrasion (hard minerals are embedded into the stylus, and then move together with the stylus on the rock surface) may also take place during scratching. Therefore, from the point of view of metallurgy science and tribology, this paper aims to detect the abrasive mechanisms on the applied tool steel under different scratching conditions. The results can provide a valuable reference for estimating the tool wear due to rock heating and scratching/cutting, for selecting an appropriate tool for excavating different rock types and in delivering the basis for further developments in tool production.

2 Materials and methods

2.1 Rock selection and preparation

We selected six types of rock for testing: two sedimentary rocks (sandstone and greywacke), three igneous rocks (diorite, granite and porphyry), and one metamorphic rock (gneiss).

We used thin section analyses to investigate the mineralogical composition and microstructure of the rock (Fig. 2). Sandstone is a fine- to medium-grained sedimentary rock. The grain sorting is moderately sorted, the grain rounding varies from sub- to well-rounded, and the grain bonding exhibits predominantly a grain-grain contact. This sandstone is composed almost of quartz. Greywacke is a finer-grained sedimentary rock. This rock has a poor-sorted and fragmented texture and consists mainly of quartz, feldspar and mica. Diorite is a medium-grained igneous rock. Plagioclase has a twinning structure with dark-green pyroxene. Granite is characterized by its medium-grained structure and consists of the typical minerals quartz, feldspar and biotite. Coarse-grained porphyry shows a porphyritic texture, in which large feldspar grains are dispersed in a fine-grained quartz-mica matrix. The Gneiss is medium-grained with regular and undulating foliation. It consists of quartz, feldspar and biotite. In addition, we performed X-ray diffraction (XRD) analyses to determine the content of constituent minerals within the rock (Fig. 3), which confirms the thin section observations. The major constituent mineral in sandstone and greywacke is quartz. The three crystalline rocks (granite, porphyry and gneiss) are mainly composed of feldspar and quartz, and the major minerals in diorite are feldspar and pyroxene.

Fig. 2
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Thin section analyses for a Sandstone; b Greywacke; c Diorite; d Granite; e Porphyry; f Gneiss

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Fig. 3
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XRD analyses for a Sandstone; b Greywacke; c Diorite; d Granite; e Porphyry; f Gneiss

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We chose eight temperature levels for heating: 25 °C (ambient temperature), 100 °C, 300 °C, 400 °C, 500 °C, 600 °C, 800 °C and 1000 °C. Rock samples were heated in an electric furnace at a constant heating rate of 1 °C/min until the desired temperature is reached. After that, the temperature was held for 2 h to obtain a homogeneous temperature distribution inside the sample. Finally, the sample was naturally cooled down to the ambient temperature in the furnace. Note that rapid temperature changes, characterized by a strong thermal gradient, have the potential to induce significant stress/strain variations in the object, resulting in thermal shock. Such abrupt stress changes can surpass the material strength, leading to the formation of cracks. A heating rate of less than 10 °C/min is thus recommended to prevent thermal shock. The slow heating rate of 1 °C/min eliminates the potential influence of the heating rate on the test results (Fellner and Supancic 2009, Wang and Konietzky 2020).

2.2 Cerchar scratch test

The Cerchar scratch test was developed in the 1980s (Cerchar 1986). We used the West apparatus (West 1989) (Fig. 4a). The applied stylus is made of 115CrV3 tool steel. Table 1 lists the chemical compositions of the stylus. The stylus is heat-treated to the Rockwell hardness of HRC 54–56. All tested rock samples are sawn to avoid the additional influence of surface roughness on the test results. Test setup and procedure are according to Alber et al. (2014). Under a 7 kg normal load, a 90° conical stylus slides 10 mm over a rock surface within 10 s (scratching velocity of 1 mm/s). The tip wear flat measured in millimeters multiplied by 10 is defined as the Cerchar abrasivity index (CAI), indicating the rock abrasivity (Fig. 4b). For each tested rock at each temperature level, five single scratches were performed to obtain the mean CAI value.

$$CAI=10\cdot d$$
(1)

where d [mm] is the diameter of wear flat on the stylus tip.

Table 1 Chemical compositions of applied stylus
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Fig. 4
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a Cerchar apparatus (West type); b Measurement of the wear flat on the stylus tip

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3 Results and discussion

3.1 Surface color and material integrity

We analyzed the surface colors of rock samples after heat treatment. The surface colors vary according to the temperature levels (Fig. 5). The sandstone changes its color from beige at 25 °C gradually into yellowish at 1000 °C. The color of greywacke is dark gray at 25 °C and turns into light gray at 500 °C, and finally into dark red at 1000 °C. When the temperature exceeds 800 °C, thermal-induced macro-cracking is found on the surface of the greywacke sample. The surface color of diorite varies from gray at 25 °C to reddish at 1000 °C. The surface color of granite, porphyry and gneiss changes from light red at 25 °C gradually into reddish at 1000 °C, and more micro-cracks are randomly generated across the sample at this temperature. The change of colors can be attributed to the transition of iron elements from low to high valence state at elevated temperatures. Moreover, when temperature reaches 1000 ºC, more micro-cracks become visible on the sample surface. Our results are in close agreement with previous studies for sandstone (Rathnaweera et al. 2018, Li et al. 2020) and granites (Liu et al. 2015).

Fig. 5
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Surface color of rock samples exposed to different temperatures for a Sandstone; b Greywacke; c Diorite; d Granite; e Porphyry; f Gneiss

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It is evident that heating-induced macro-cracks develop only in samples subjected to very high temperatures, such as 800 °C and 1000 °C. This aligns well with findings of a previous study (Wang et al. 2021b). The macro-cracks observed on the rock surface are the result of the development, accumulation and coalescence of thermal-induced micro-cracks. Various rock types, including sandstone and granite, exhibit a progressive increase in thermal-induced damage at elevated temperatures. At high temperatures like 800 °C and 1000 °C, the initially induced cracks undergo accumulation or widening due to the influence of temperature and mineral phase transitions, signifying a substantial loss of material integrity.

3.2 Rock quality

We performed ultrasonic measurements to determine the P-wave velocity (Vp) to assess the rock quality at elevated temperatures.

Figure 6 shows the P-wave velocity plotted against the corresponding temperature for each tested rock. It is reasonable that, at ambient temperature of 25 ºC, the highest velocity is found for fine-grained sedimentary greywacke, followed by medium-grained intrusive diorite and the three medium- to coarse-grained crystalline rocks of granite, gneiss and porphyry. The medium-grained sedimentary sandstone has the lowest velocity due to its higher porosity. Compared to 25 ºC, a slight increase of P-wave velocity is found for diorite and the three crystalline rocks at 100 ºC, but its value decreases for greywacke and sandstone, meaning that the heat treatment may have a stronger influence on sedimentary rocks. Starting from 100 ºC, a clear decreasing trend of P-wave velocity with increasing temperature is identified for all the tested rocks. This confirms the conclusions of previous studies for sandstone (Liu et al. 2015; Sun et al. 2017) and granitic rocks (Liu et al. 2015; Zhu et al. 2017; Wang et al. 2020; Ahmed et al. 2022), meaning that P-wave velocity is affected by the material damage due to rock heating. More interesting to see is that the P-wave velocity decreasing rate is greater in the temperature range between 25 and 600 ºC than that between 600 and 1000 ºC. It might be that, below 600 ºC, primarily new but isolated cracks develop within the rock, and these cracks connect when the temperature exceeds 600 ºC, whereas the generation of new cracks is then decelerating.

Fig. 6
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Evolution of P-wave velocity with temperature

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3.3 Rock abrasivity

CAI values for the tested rocks at different temperatures are summarized in Table 2. Figure 7 shows the CAI value plotted against the corresponding temperature. Three trends can be identified. For the two quartz-dominant sedimentary rocks (sandstone and greywacke), CAI values first increase slightly with increasing temperature up to 500 °C, and then decrease continuously until 1000 °C. This behavior can be attributed to the α-β-quartz transition at temperature of 573 °C. After that, clear changes appear due to the development of micro-cracks at grain boundaries at temperature above 600 °C. Moreover, thermal-induced cracking appears not only along the grain boundaries, but also inside the quartz grains at temperature above 750 °C. Clay minerals (e.g., kaolinite and chlorite) are also sensitive to heating. After heat treatment, clay minerals experience a series of physical and chemical changes including evaporation of water and decomposition of mineral composition, which causes the destruction of mineral structure. The release of interlayer water from clay minerals as a result of heating is called dehydroxylation. For example, the dehydroxylation of kaolinite occurs at 400–800 °C (Zhang et al. 2020c), and the kaolinite structure collapses completely at about 550 °C (Hajpal and Török 2004). Several clay minerals decompose at high temperatures, and these reactions cause a significant damage of crystal lattice, and further on cause an increase in defects of clay minerals. Shrinkage cracks appear in layered minerals and phyllosilicates (e.g., biotite and muscovite) at lower temperatures up to 450 °C. For the three quartz-rich crystalline rocks (granite, porphyry and gneiss) the evolution of CAI shows a similar trend: CAI values remain more or less constant up to about 500–600 °C, and then decrease continuously until 1000 °C, mainly caused by the property change of quartz minerals at elevated temperatures. It is remarkable that, for quartz-less diorite, CAI values show only a very weak fluctuation with increasing temperature up to 800 °C, and CAI has the highest value at 1000 °C. This behavior may be caused by the compaction and crack closure induced by thermal expansion of mineral grains (e.g., feldspar). The reduction of pre-existing cracks due to partial melting and recrystallization of minerals may also be a reason (Ji et al. 2020). He et al. (2018) measured the content of three major minerals within a granite sample at different temperatures by using XRD. Results showed that the diffraction intensity of quartz does not show a large change with increasing temperature up to 500 °C, but shows a drastic decrease until 700 °C. The intensity decrease is more than 60%. The maximum diffraction intensity of feldspar (i.e., plagioclase) is found at 300 °C, and the intensity then decreases by about 37% at 700 °C. The diffraction intensity of mica decreases continuously from 20 to 700 °C, with a decrease of 25%. The authors concluded that the phase transition of minerals is the main reason for the strength deterioration of the rock due to heating. Taking sandstone as an example, the UCS of sandstone first increases with increasing temperature up to 400–600 °C, and then decreases until 1000 °C (Wang et al. 2021a). The evolution of CAI shows a good agreement with that of UCS at elevated temperatures.

Table 2 Summary of CAI values for tested rocks at different temperatures (mean value of five single tests)
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Fig. 7
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Evolution of Cerchar abrasivity index with temperature (colored dots indicate single value; dashed line connects mean values)

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To determine the change of CAI due to heating, a normalized value calculated as the ratio of CAIT to CAI0 is proposed, where CAIT is the CAI value at a given temperature and CAI0 is the CAI value at ambient temperature of 25 °C. Such normalization offers a better comparison. Figure 8 shows normalized CAI values versus temperature and corresponding depth. The ambient temperature is assumed to be 25 °C on the surface, and the average temperature-depth gradient is approximately 25 °C/km. For sandstone and greywacke, the normalized CAI increases with increasing temperature, and this value is about 1.5 times higher at 500 °C compared to 25 °C. Afterwards, the ratio of CAIT/CAI0 decreases gradually to about 0.4–0.6 at 1000 °C. For granite, the normalized CAI is kept more or less constant at 1.2 in the temperature range between 100 and 600 °C, and then experiences a clear decrease (only about 0.2 at 1000 °C compared to 25 °C). For porphyry and gneiss, the CAIT/CAI0 value remains almost unchanged up to 500 °C, and then decreases with increasing temperature to about 0.4 at 1000 °C. For diorite, the CAIT/CAI0 value does not show a significant change until 600 °C, but increases by about 0.2 until 1000 °C is reached. The change of CAI, or more exactly, of rock abrasivity can be divided into two stages on either side of 500 °C.

Fig. 8
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Normalized Cerchar abrasivity index versus temperature and depth

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By comparing the reservoir and treatment temperatures, our result shows a good agreement with that of Ji et al. (2020) for granite. Moreover, it is found that a typical reservoir temperature (150–200 °C) has a more significant influence on CAI and rock abrasivity of sedimentary rocks compared to crystalline rocks.

3.4 Material removal volume

We measured the material volume (Vm) removed from the rock surface by using a digital microscope (Fig. 9), at temperatures of 25 °C, 300 °C, 500 °C, 800 °C and 1000 °C, respectively. This microscope can quickly scan through the focal range of a sample to build a fully focused image. Even when the sample surface has a significant variation in height, the focused image can be obtained instantly by compiling images at different focal planes. After creating the composite image, focal position data are collected to construct a three-dimensional model, and after constructing the model, data are used to calculate the volume and height of the area within the view field (Zhang et al. 2020b).

Figure 10 shows the determined average material removal volume, Vm, measured by three scratches for each rock sample at each given temperature. An increasing trend of Vm with increasing temperature is found for the two sedimentary rocks (sandstone and greywacke). Taking sandstone as an example: Vm increases from 4.4 to 10.9 mm3 at 300 °C, then decreases slightly to 7.0 mm3 at 500 °C, and finally increases with increasing temperature to 27.2 mm3 at 800 °C and 76.7 mm3 at 1000 °C. The evolution of Vm for the three crystalline rocks (granite, porphyry and gneiss) is similar. For example: Vm for granite is 0.56 mm3 at 25 °C, 1.24 mm3 at 300 °C, 0.23 mm3 at 500 °C, 5.14 mm3 at 800 °C and 15.26 mm3 at 1000 °C. However, Vm for diorite has no clear correlation with temperature. It can be said that rock heating leads to increasing removal of material from the rock surface due to the reduction of rock hardness and strength.

Fig. 9
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a Digital microscope used to measure b the material volume removed from the rock surface of a sandstone sample

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Fig. 10
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Evolution of material removal volume with temperature (mean value of three scratches)

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3.5 Drilling efficiency

We evaluated the drilling efficiency by means of the Cerchar abrasion ratio (CAR), which is defined as the ratio of material removal volume (Vm) to the tip wear volume (Vs), according to Eq. 2 (Zhang and Konietzky 2020). Note that CAR can also be used to study the interaction between rock and tool, because it relates the two volumetric parameters.

$$CAR= \log_{10}(\frac{{V}_{m}}{{V}_{s}})$$
(2)
$${V}_{s}= \frac{1}{3}\pi {(\frac{d}{2})}^{3}$$
(3)

Figure 11 shows the CAR value plotted against the corresponding temperature for each tested rock, and Fig. 12 shows the normalized CAR for comparison. For sandstone and greywacke, CAR first shows a slight decrease with increasing temperature up to 500 °C, and then shows a clear increase until 1000 °C. The corresponding drilling efficiency at 1000 °C is about 1.6 times higher than that at 25 °C. The evolution of CAR for granite and porphyry at elevated temperatures shows a similar trend. For these two rocks, the drilling efficiency at 1000 °C is about 2.7 times that at 25 °C. A continuous increase of CAR with increasing temperature is found for gneiss, meaning that the drilling efficiency is also enhanced (about 1.8 times higher at 1000 °C compared to 25 °C). However, for diorite, pro-heating of rocks seems to have no significant influence on the drilling efficiency. In general, below 500 °C, no clear enhancement of drilling efficiency is identified with increasing temperature. However, when temperature exceeds 500 °C, the drilling efficiency becomes clearly greater, especially for quartz-rich rocks with large mineral grains.

Fig. 11
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Evolution of Cerchar abrasion ratio with temperature

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Fig. 12
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Normalized Cerchar abrasion ratio versus temperature

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3.6 Damaged rock surface

We investigated the damaged surfaces after rock scratching by using a scanning electron microscope (SEM). This investigation offers a better insight how scratch grooves are formed and rock materials are removed by rock indenting and scratching at elevated temperatures (Piazzetta et al. 2018).

Figure 13 shows SEM micrographs of scratch grooves of the tested rocks at temperatures of 25 °C, 500 °C and 1000 °C, respectively. By investigating the scratch grooves on sandstone, no significant difference is found between 25 °C, 500 °C and 1000 °C. In all cases, the groove bottom is rough and the groove edges vary, which can be attributed to the lower strength and higher porosity of sandstone. After the stylus moved over the rock, a large amount of wear debris is detached from the rock surface due to crack formation and propagation, and thus micro-cracking is the dominant wear mechanism. The shape of scratch grooves on greywacke is quite different at 500 °C and 1000 °C, compared to 25 °C. The groove bottom is relatively smooth at ambient temperature. Fine-grained minerals are crushed into finer debris due to high contact stress, and then are removed by the stylus. It seems that micro-cutting is the dominant wear mechanism. However, when the temperature reaches 500 °C and 1000 °C, the groove bottom becomes much rougher. The depth of scratch is greater than that at 25 °C, meaning that more wear debris is removed. In these two cases, a micro-cracking wear mechanism may be dominant. The scratches on the diorite show more or less a similar behavior at the three temperatures. At 25 °C and 500 °C, respectively, a shallow groove is formed, and scratching to grinding abrasion is the dominant wear mechanism. In comparison, at 1000 °C, the depth of scratch is increased, and a clear worn surface with more debris removal occurs. A transformation from grinding to gouging wear mechanism may take place. The scratch grooves are almost the same for both, medium-grained granite and coarse-grained porphyry at temperatures of 25 °C, 500 °C and 1000 °C, respectively. Before heat treatment and at 500 °C, scratching and grinding abrasion occurs due to the high strength of the rock. After the rocks experienced heating up to 1000 °C, thermal-induced cracking can be identified on the surface of the two granitic rocks. Width and depth of the scratches are enlarged. The worn surface exhibits a damaged surface associated with inter-granular fracturing, trans-granular fracturing and pull-out of grains, and gouging abrasion is the dominant wear mechanism. Although the scratches on gneiss are similar for temperatures of 25 °C, 500 °C and 1000 °C, the wear mechanism occurring at the three temperatures may be different. Flattened debris is found on the groove bottom at 25 °C and 500 °C, which indicates that grinding abrasion may take place. At 25 °C, the enlarged groove edges can be attributed to the stylus scratching over layered mica minerals. The surface damage is most likely caused by a combined effect of micro-cutting and -cracking. In contrast, the grooved interior is free of wear debris at 1000 °C. The chaotic groove edges reflect that the wear mechanism is predominantly caused by micro-cracking. After analyzing the SEM morphologies of scratch grooves, we can deduce that thermal-induced cracks initiate and propagate on and beneath the stressed rock surface. The depth of scratch groove is increased at elevated temperatures due to strength deterioration of the rock. The material removal can be related to the depth of the scratch. The deeper the scratch groove, the more material is removed (Fig. 13).

Fig. 13
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SEM micrographs of scratch grooves at temperatures of 25 °C, 500 °C and 1000 °C for a sandstone; b greywacke; c diorite; d granite; e porphyry; f gneiss

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3.7 Worn steel surface

We analyzed the worn steel surfaces by using SEM to investigate the temperature effect on the wear formation on the stylus tip after rock scratching.

Figure 14 shows SEM morphologies as consequence of a combined sliding and impact wear on the steel surface in contact with rocks which experienced temperatures of 25 °C, 500 °C and 1000 °C, respectively. In most cases, the worn steel surface shows severe plastic deformations due to sliding on hard and abrasive minerals (i.e., quartz) under a high contact stress. The abrasive wear leads to removal of steel material as well as initiation and propagation of cracks associated with delamination, dislocation and chipping of the steel. Narrow or wide, shallow or deep furrows or grooves filled with or without rock debris regular or chaotic to the sliding direction of the stylus are the wear characteristics. These can be related to the following mechanisms: first, harder and more abrasive minerals like quartz can easily cut the steel, which results in deeper wear grooves on the steel surface. Abrasive wear loss is predominantly caused by grooving and micro-cracking, and micro-cracking leads to flaking. Second, abrasive minerals like feldspar and plagioclase, whose hardness is approximately equal to steel, are only partially able to penetrate or cut the steel. Steel materials are detached from the wearing surface by fracturing after plastic deformation of the stressed surface. Third, less abrasive and softer minerals like mica or clay minerals cannot cut the steel. Wear loss is predominantly caused by removal of the steel binder phase on the wearing surface due to plastic deformation (Zum Gahr 1987; Gate 1998). Another typical wear appearance on the worn steel surface is a large amount of rock debris (either ground or crushed) at the front side of the stylus indicating the sliding direction of the tool against the rock, as well as debris embedded in the steel surface, reflecting a continuous or repeated impact action of the stylus on the rock. The amount of embedded particles can be related to the hardness of individual minerals. Wear in form of surface fatigue may also occur. Shallow or deep gouging pits of different sizes between or accompanied with the grooves are visible on the steel surface (Pintaude et al. 2009; Wimmer et al. 2001). They are most likely formed by detached abrasive particles, which indented into the steel matrix during sliding, or they represent those areas, in which steel has been fragmented. Delamination and dislocation of steel is also observed due to plastic deformation of the surface layer caused by the slider (Jahanmir et al. 1975).

Fig. 14
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SEM micrographs of stylus tips at temperatures of 25 °C, 500 °C and 1000 °C for a sandstone; b greywacke; c diorite; d granite; e porphyry; f gneiss

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4 Summary and conclusions

In this paper, the sensibility of rock abrasivity to temperature as well as the enhancement of drilling efficiency due to pre-heating of the rock is documented based on six types of rock by using the Cerchar testing method.

Results of CAI verify that rock abrasivity is temperature-dependent. The evolution of CAI can be divided into two stages on either side of 500 °C. The change of properties can be attributed to the phase transition of constituent minerals at elevated temperatures. Note that, besides abrasivity, hardness and strength are also of vital importance for rock excavation and tool wear, and all these three parameters as well as their relationships at elevated temperatures will be studied in future work.

Results of CAR reflect that drilling efficiency is significantly affected by pre-heating of rocks. Especially when the heating temperature exceeds 500 °C, drilling efficiency can clearly be enhanced for quartz-rich rocks, at least by a factor of 1.6. Hence, we can estimate tool wear and evaluate drilling efficiency with the aid of the Cerchar testing method. However, experimental results presented within this paper need to be verified further by in-situ cutting or drilling tests.

Looking to the abrasive wear occurring on rock surfaces, the damaged rock surface is mainly characterized by grooves. Rock materials are removed under the combined effect of micro-ploughing, -cutting and -cracking, depending on rock types. Compared to ambient temperature, more materials are removed from sample surfaces which experienced heating, reflecting that heating leads to a reduction of rock hardness and strength.

In respect to tool wear, two- and three-body abrasions occur during Cerchar scratching process. The worn steel stylus surfaces show severe plastic deformation, which further on causes fracturing. The abrasive wear leads to the removal of steel material and the initiation and propagation of cracks on the steel surface. The dominate wear mechanisms are grooving and micro-cracking. More studies have to be performed to investigate the effect of steel type on its wear formation.