Mechanical Behavior andPerformance of Ceramics

and Composites

COPYRIG

HTED M

ATERIAL

3

THE EFFECTS OF DIAMOND GRIT CHARACTERISTICS ON THE MICROSTRUCTURE AND ABRASION RESISTANCE OF PCDs SINTERED BY HPHT

Lifen Deng, Roger Nilen, Serdar Ozbayraktar Global innovation centre, Element Six Ltd. Harwell, Oxfordshire, UK

ABSTRACT

Polycrystalline diamond (PCD) compacts used on oil and gas drilling bits were sintered under high-pressure and high-temperature (HPHT) conditions. In this study, the microstructure of polycrystalline diamond composite portion of the compacts is analysed by using scanning electron microscopy (SEM) and image analysis methods. In addition, laboratory rock cutting tests were conducted as a direct measure of abrasion resistance of PDCs produced using different types of diamond powder. Microstructure and abrasion resistance relationships with the grain size, the nitrogen content and the surface quality of diamond grits are discussed. It is found that the PCDs’ abrasion resistance generally decreases with the increasing mean diamond grain size of a sintered PDC. Although the diamond grits with lower nitrogen levels show a higher compression fracture strength, it is not necessarily beneficial for the abrasion resistance of PCDs. It was observed that Type C diamond grits with a rougher surface tends to lead an increasing abrasion resistance of PCD in laboratory rock cutting tests.

INTRODUCTION

Polycrystalline diamonds (PCDs) sintered by HPHT method have been widely used in oil and gas industry as the cutting element on the drilling bit since 1980’s and polycrystalline diamond compact drill bits have accounted for over 60% of drilled footage in 2004[1]. Due to the PCD’s super abrasion resistance and reasonably good fracture toughness resistance compared to traditional cutting element materials such as tungsten carbide and hardened steel, PCD compacts have significantly increased the drilling footage and reduced the cost in oil and gas industry [2]. As conventional oil fields are becoming increasingly elusive and continuously reducing the drilling cost is always in demand, the drive for further enhanced abrasion resistance of PCD cutters but not sacrificing too much the fracture toughness is stronger than ever. In the present study, the relationship between the abrasion resistance of PCD cutters and the grain size was discussed. At the same time, 3 different types of Grade 30 (G30, the mean grain size = 30

m) and Grade 22 (G22, the mean grain size = 22 m) diamond grits were used to sinter PCD cutters. Type A diamond powders have a higher nitrogen content than Type B and Type C powders. The surface of Type C diamond grits are rougher than Type B. All PCD cutters were sintered under the same conditions. How the abrasion resistance of PCDs is affected by the nitrogen content and the surface quality of diamond grits was investigated.

EXPERIMENTAL

The nitrogen content of diamond grits were measured by a LECO Nitrogen Analyzer. Each sample was measured 4 times and the average value calculated. In order to investigate the compaction response of diamond grits at high pressure, a cold compaction test was conducted on the three types of G22 and G30 diamond powders. The diamond powders were put into a metal cup and topped with a carbide substrate for the cold compaction at 7GPa and 8GPa at room

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temperature. The particle distribution of diamond grits were measured by Malvern Particle Master Analyzer before and after the cold compaction.

The PCD cutters were sintered under conditions of 1400 to 1600 °C and 5 to 8GPa [3,4]. Figure 1 shows the schematic design of the pre-sintered unit and the microstructure of PCDs and the tungsten carbide after sintering. During the sintering of PCD, the cobalt in substrate becomes melted and infiltrated into the bed of diamond grits, assisting the sintering of diamonds.

After sintering, all PCD cutters in the present study were processed to the same dimensions of Ø16mm × 12mm. The abrasion resistance of PCD was measured through rock cutting tests conducted on vertical turret lathe. After the test, the wear scar were measured under an optical light microscopy. The larger the wear scar area of PDC, the lower the wear resistance. For each sample, 4 rock cutting tests were carried out on two locations of 2 PCD cutters, generateing a total 4 data points.

Figure 1: The schematic picture showing the sintering of PCDs.

All PCDs were lapped and polished to 1 m before the microstructure observation under SEM. With the intention of quantify the microstructure of PCDs, 20 photos were taken on each sample and processed by the commercially available image analysis software package. The percentage of diamond area is one of the image analysis results of PCDs’ microstructure and used to present the sintering status of PCDs. Given the same powder feed, the higher the percentage of diamond area, the more PCD sintered.

RESULTS AND DISCUSSION

Mean grain size of diamond grits A series of diamond powders with different average grain sizes were prepare and sintered

into PCD cutters at 7 GPa. All PCDs were sintered under the same conditions. The SEM observations confirmed that the microstructures of all PCDs showed uniform diamond grain distribution and sintered well. Figure 2(a) and (b) show the microscopy photographs of PCDs with relatively fine and coarse grain sizes. The dark parts are diamond grains and the white dotted boundaries are cobalt binders. It is clearly observed that diamond grains are bonded to each other with some cobalt residual existing at the grain boundaries.

Diamond grits

WC

Cobalt

WC

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Figure 2: Microscopy photographs of PCDs with (a) fine and (b) coarse grain size.

(Magnification: ×1000)

It is found that the wear scar area generally increases with the grain size of diamond grits although a linear relation is not strictly followed. In general, a negative correlation is observed between the abrasion resistance of PCDs and the grain size of diamond grits.

Nitrogen content and surface quality of diamond grits Figure 3 shows the average nitrogen contents of three different types of G30 and G22

measured using LECO Nitrogen Analyzer. It is shown that Type A diamond powders have a higher nitrogen content than Type B and Type C. For G30, the nitrogen level of Type B powders is slightly higher than Type C while for G22, the nitrogen content of Type B and Type C are nearly same.

Figure 3: The average nitrogen content of three types of G30 and G22 diamond grits.

Figure 4(a) and (b) show the SEM images of G30 diamond grits (Type A) before and after the compaction at 7GPa. It is observed that the original, generally uniform coarse diamond grits were smashed into smaller grits with few relatively coarse ones. The original diamond powders have a relatively clear and smooth appearance while after compression, the smashed diamond grits showed sharp edges and rough surfaces. Obviously, the crushing occurred during the high pressure compaction helped the further dense of the diamond bed before the sintering with the infiltration of liquid cobalt. Figure 5(a) clearly displayed the particle distribution shifted after the compaction at 7GPa.

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G30

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Type A Type B Type C

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G22

(a) (b)

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Figure 4: (a) The photo of original G30 diamond grits (Type A) before the press. (b) The photo of thesame diamond grits after compaction at 7GPa. (Magnification: ×500)

Figure 5(b) and (c) are the particle distribution plots of G30 diamond grits after the compaction at 7GPa and 8GPa, respectively. Figure 5(b) clearly disclosed that the crushing rate of Type A diamond grits is higher than Type B and C. It does indicate that the diamond grits (Type B and C) with lower nitrogen contents exhibited a high compression fracture strength than Type A ones, which is agree with the literature [5]. Brookes and Daniel reported that [5] at room temperature, a reduction in the concentration of single substitutional diamond increases the indentation hardness of the diamond. Figure 5(c) showed that the particle distribution plots of G30 Type A and Type B are nearly same after the compaction at 8GPa. Type C ones share the same particle distribution as Type A and Type B as well. Just for a clear appearance of the plots, the data of Type C is not included in Figure 5(c). It means that the advantage of higher compression

Figure 5: Particle size distribution plots of G30 (a) before and after compaction, (b) after compaction at 7GPa, (c) after compaction at 8GPa. (d) Particle size distribution plots of G22 after compaction at 7GPa.

0

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Vol%

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(a) G30 before and after compaction at 7GPa

After…Before…

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(b) G30 after compaction at 7GPaType AType BType C

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(c) G30 after compaction at 8GPa

Type AType B

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(d) G22 after compaction at 7GPaType AType BType C

(a) (b)

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fracture strength of Type B and C diamond powders due to the lower nitrogen content disappeared when the pressure increased from 7GPa to 8GPa. Figure 5(d) showed the particle distribution plots of G22 diamond grits after compaction at 7GPa. The 3 different types of diamond grits shared a very similar particle distribution except Type C showed a marginally lower crushing rate than the other two types. It means that for G22 diamond, type B and C loose the advantage of higher compression fracture strength at lower pressures (say 4-5GPa but no data available) and hence exhibit a similar grain size distribution at 7GPa for all three types. Type C powders appeared to have a rougher surface s associated with reduced stress concentration sharp edges after an etching heat treatment, which might explain the slightly lower crushing rate if Fig. 5(d) is accurate. However, how the Type C powders’ surface quality affect the cold compaction, hot compaction and following diamond sintering is not clear yet. Because all three types of G22 diamonds already crushed similarly at 7GPa, it is reasonable to believe that they would crush similarly at 8GPa. Therefore no cold compaction test is carried out at 8GPa.

Figure 6(a) (b) and (c) showed the microstructures of PCDs sintered from the three different types of G30 diamond grits. All PCD microstructures are uniform and look very similar. Figure 6(d) is the percentages of diamond areas extracted from the image analysis. Generally speaking, the three types of PCDs showed a similar diamond area percentage except that Type C one had a slightly lower diamond area.

Figure 6: (a) (b) and (c) Microscopy photographs of PCDs individually sintered from G30 Type A, B and C diamond grits with the magnification of ×1000. (d) The percentages of diamond area in the microstructure of PCDs sintered from G30 diamond grits.

Type CType BType A

95.0

94.5

94.0

93.5

93.0

Dia

mon

d ar

ea (

%)

93.925494.0188

93.5311

Boxplot of Type A, Type B, Type C

(a) (b)

(c) (d)

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Figure 7(a) (b) and (c) are the microstructure of PCDs sintered from the three different types of G22 diamond grits. They are uniform and look very similar as well. Figure 7(d) showed that the percentages of diamond area of PCDs from Type A and Type B are very similar, both lower than that from Type C.

Figure 7: (a) (b) and (c) Microscopy photographs of PCDs individually sintered from G22 Type A, B and C diamond grits with the magnification of ×1000. (d) The percentages of diamond area in the microstructure of PCDs sintered from G22 diamond grits.

Figure 8 (a) and (b) showed the wear scar areas of PCDs sintered from G30 and G22 at 8GPa, respectively, after the vertical turret test. It turns out that although Type B diamond grits have higher compression fracture strength than Type A due to the lower nitrogen content, the abrasion resistance of PCDs is not necessarily improved. Actually, the abrasion resistance tested in this study decreased for both G22 and G30 as the diamond grits changed from Type A to Type B. The reason is unclear yet. It may rely on the vertical turret test which might be more sensitive to the grain boundary rather than the diamond grit itself. Or it may because less plastic deformation occurred at high temperature in diamond grits with lower nitrogen content, therefore the sintering process is affected accordingly. According to [5], when the temperature is high enough (1400 C), an increase in nitrogen content is reflected in a decrease in the volume of plastic deformation associated with impressions. On the other hand, bearing in mind that after HPHT sintering, the total nitrogen level in PCDs changed. At HPHT conditions, nitrogen atoms become mobile in diamond lattice, therefore the original internal nitrogen interstitial can diffuse outside and the external nitrogen could enter diamond as well. However, it seems to suggest that Type C powders might be beneficial for the improvement of abrasion resistance of PCDs whatever the percentage of diamond area is higher or lower. The reason might be related to the increased surface area due to the enhanced roughness of diamond grit and also to the potential

Type CType BType A

95.0

94.5

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d A

rea

(%)

93.2024 93.2169

94.1534

Boxplot of Type A, Type B, Type C

(a) (b)

(c)

(d)

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more active surface on the diamond grits after the etching process. The further study is needed for the surface quality of diamond grits and how they affect the bonding between diamond to diamond during sintering.

Figure 8: Plots of wear scar areas of PCDs sintered from 3 different types of G30 and G22 diamond grits at 8GPa.

CONCLUSIONS

Considering the limited data and the complication of PCD cutters’ abrasion performance, it is too early to draw some solid conclusion. However, the main findings from the present study can be summarised as below.

1) The abrasion resistance of PCDs generally decreases as the grain size increases.

2) Type B and Type C diamond grits show lower nitrogen content which could help G30 to crush less under 7GPa but not at under 8GPa.

3) For G22, Type B and Type A diamond grits crushed similarly under 7GPa. Type C diamond grits crushed slightly less than Type B under 7GPa, which might be attributed to the higher roughness of diamond grit surface.

4) Although diamond grits of Type B with lower nitrogen contents are harder than Type A ones, it is not necessarily beneficial for the improvement of PCD’s abrasion resistance.

5) Type C diamond grits of both G22 and G30 seem to help improve the abrasion resistance of PCDs, which might be due to the increased surface area of diamond and the potential enhanced activeness during sintering.

REFERENCES

[1] Scott 2006, The history and impact of Synthetic Diamond Cutters and Diamond Enhanced Inserts on the Oil and Gas Industry

[2] Valentine Kanyanta, Hard, super hard and ultrahard materials: An overview, Microstructure-Property Correlations for Hard, Superhard, and Ultrahard Materials, © Springer International Publishing Switzerland 2016, P15

[3] Robert Fries, Patent number 9120204, Polycrystalline superhard material

[4] J. J. Barry, T. P. Mollart, R. W. N. Nilen, patent number 9095918, Cutter structures, inserts comprising same and method for making same

[5] E.J. Brookes, R.D. Daniel, Influence of nitrogen content on the mechanical properties of diamond, Properties, growth and applications of diamond, published by: INSPEC, 2001.

Type CType BType A

13

12

11

10

9

8

Wea

r sc

ar a

rea

(mm

2)

10.395

11.525

9.67

Interval Plot of Type A, Type B, Type C95% CI for the M ean

Individual standard deviations were used to calculate the intervals.

Type CType BType A

12

11

10

9

8

7

Wea

r sc

ar a

rea

(mm

2)

8.6925

11.4825

10.1375

Interval Plot of Type A, Type B, Type C95% CI for the Mean

Individual standard deviations were used to calculate the intervals.

(a) G30 _8GPa (b) G22 _8GPa