Development of Self-lubricating Ceramic Cutting Tool

Development of Self-lubricating Ceramic Cutting Tool Material with the

Addition of Metal Coated Solid Lubricant Powders

WU Guangyong1, a, XU Chonghai1, 2, 3,b, XIAO Guangchun2, 3,c, YI Mingdong2, 3,d, CHEN Zhaoqiang2, 3,e

1School of Mechanical Engineering, Shandong University, Jinan 250061, China 2School of Mechanical and Automotive Engineering, Qilu University of Technology, Jinan 250353,

China 3Key Laboratory of Advanced Manufacturing and Measurement and Control Technology for Light

Industry in Universities of Shandong, Qilu University of Technology, Jinan 250353, China [email protected], [email protected], [email protected], [email protected], [email protected]

Keywords: Self-lubricating; Ceramic cutting tool; Electroless plating; Ni-coated h-BN

Abstract. A new type of self-lubricating ceramic cutting tool material with the addition of pure

metalcoated solid lubricant powders was developed. Electroless plating technique was applied to

produce pure nickel coated h-BN composite powders with core-shell structure. Self-lubricating

ceramic cutting tool material containing the as-prepared composite powders was prepared by hot

pressing. It exhibited notable improvements in microstructure, mechanical properties and cutting

performance, in comparison with the corresponding cutting tool material made by directly adding

uncoated h-BN powders. The new type of self-lubricating ceramic cutting tool material may have a

promising prospect in the field of high speed dry machining.

Introduction

In mechanical manufacturing, traditional machining (such as turning and milling) is the most

fundamental and reliable processing method. The cutting tool performance is one of the decisive

factors that affect the machining efficiency, accuracy and surface quality. Cutting tool material has

generally experienced several developmental stages, including carbon tool steel, high speed steel,

cemented carbide, ceramic and super hard material. As one of the most important advanced

manufacturing technologies, high speed machining has attracted more and more attention nowadays

because it can increase productivity and reduce production costs dramatically [1]. Compared with

conventional tool materials such as high speed steel and cemented carbide, ceramic cutting tool

materials show unparalleled advantages in the field of high speed machining, due to their excellent

performance in terms of high hot hardness, chemical inertness, and thermal stability[2].

Owing to high cutting temperature and large cutting force in high speed machining, wear of the

ceramic cutting tool is comprehensively resulted from mechanical and chemical effects. Its wear

mechanism mainly involves abrasive wear, adhesive wear, chemical reaction, diffusion wear and

oxidation wear [3,4]. Appropriate cooling and lubrication can be of great benefit to reducing wear of

ceramic cutting tools and prolonging their service life. The traditional cutting fluid lubricating method

not only leads to high cost and environmental pollution, but also hardly takes effect at the tool-chip

interface. In addition, using cutting fluid may cause hot cracks and breakage in ceramic cutting tools

owing to their poor thermal shock resistance. Therefore, development and application of

self-lubricating ceramic cutting tool is undoubtedly one of the best ways to solve this problem [5]. It is

a high efficient and pollution-free lubricating method with a great potential in high speed machining.

The existent self-lubricating ceramic cutting tool materials are made by sintering ceramic

composite powders with the addition of solid lubricant. Directly adding solid lubricant can produce

two effects on properties of ceramic materials [6,7]. First of all, the solid lubricant can reduce the

friction coefficient by forming a lubricating film in the working areas. Secondly, the dispersed solid

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Proceedings of the 20th International Symposium on Advances in Abrasive Technology 3-6 December, Okinawa, Japan

lubricants can cause a decline in mechanical properties of the ceramic matrix, especially the hardness

and fracture toughness, which will reduce the wear resistance of the cutting tool materials [8]. Thus

the existent self-lubricating ceramic cutting tool materials are not available to possess rational

combination of antifriction and antiwear properties, In order to solve this difficult problem, a new

type of self-lubricating ceramic cutting tool material with the addition of metal coated solid lubricant

powders was proposed and produced in this paper.

Metal coated solid lubricant powders are composite powders with a solid lubricant core and

metallic shell. The coated powders are expected to have reinforcement effect on the new type of

cutting tool material compared with the traditional cutting tool materials. On the one hand, the

metallic shell is able to protect the solid lubricant core from decomposing and reacting with the

ceramic matrix [9], which keeps favorable self-lubricating property. On the other hand, the metallic

shell can increase overall mechanical properties [10] and then enhance wear resistance of the cutting

tool material. The two aspects synergistically endow the self-lubricating ceramic cutting tool material

with excellent antifriction and antiwear properties.

Electroless plating has been recognized as one of the most convenient and effective techniques to

synthesize core-shell composite powders with metallic shell [11]. It is an autocatalytic chemical

reduction process in which the metallic ions are reduced by the reductant and then deposited on the

substrate surface. In this paper, nickel coated hexagonal boron nitride powders (h-BN@Ni) were

prepared by ultrasonic assisted electroless plating process. Then the composite powders were mixed

with α-Al2O3 and (W,Ti)C powders to prepare Al2O3/(W,Ti)C/h-BN@Ni self-lubricating ceramic

cutting tool material by hot pressing. Its microstructures, mechanical properties and cutting

performance were studied and discussed.

Experimental Procedure

Materials and Processing.

The starting powders were commercially available α-Al2O3, (W,Ti)C and h-BN with average particle

size of 0.2 μm, 1.5 μm and 3.5 μm, respectively. Before the electroless nickel plating process, the raw

h-BN powders were firstly cleaned with 10% sodium hydroxide solution to remove impurities on the

surface and then coarsened by a hydrophilic solution (100 ml/L HF(40%) +2g/L NH4F) for 10 min.

The coarsened h-BN powders were sensitized by immersion in SnCl2/HCl solution (20 g/L SnCl2+40

ml/L HCl(37%)) for 15 min and then activated by immersion in PdCl2/HCl solution (0.5 g/L

PdCl2+10 ml/L HCl(37%)) for 5 min. Subsequently, the activated h-BN powders were filtrated and

rinsed with distilled water until the pH of the filtrated solution was 7.0, and finally dried in a vacuum

oven.

The pretreated h-BN powders were dispersed in distilled water and then poured into an electroless

nickel plating bath, whose composition and parameters are listed in Table 1. N2H4·H2O was used as

the reductant to coat h-BN with pure nickel. The overall plating reaction can be expressed as follows

[12]: 2 -

2 4 2 22Ni N H 4OH 2Ni N 4H O (1)

During the plating process, the plating solution released a mass of bubbles and its color changed from

blue to black. Ultrasonic agitation was applied to continuously provide fresh electrolytes to the h-BN

surface and prevented h-BN powders from aggregating. The pH value of the plating solution was kept

constant by adding NaOH solution. After electroless plating, the Ni-coated h-BN powders were rinsed

with distilled water for several times and then dried in a vacuum oven at 70-80 °C for 8-10 h.

The original α-Al2O3 and (W,Ti)C powders with volume ratio of 55:45 were mixed in absolute

alcohol by ultrasonic processing, with the addition of a small amount of MgO as sintering additive.

Then the powder slurries were wet milled with cemented carbide balls for 48 h. After that the

as-prepared Ni-coated h-BN powders were added into the slurries and milled for another 2 h. The

amount of the added Ni-coated h-BN powders was calculated to make the core h-BN powders account

for 5 Vol.% in the as-prepared Al2O3/(W,Ti)C/h-BN@Ni material. After milling, the slurries were

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Advances in Abrasive Technology XX

dried in vacuum and then sieved. The combined powders were hot pressed in vacuum at 1600 °C for

15 min with a pressure of 25 MPa to obtain a ceramic disk with 42 mm in diameter and 5 mm in

thickness. For comparison, an Al2O3/(W,Ti)C/h-BN ceramic disk with 5 Vol.% uncoated h-BN

powders was also prepared by using the same technical process.

Table 1. Composition and parameters of the electroless nickel plating bath

Role in the bath Chemical Parameter

Main salt Nickel sulfate (NiSO4·6H2O) 25 [g/L]

Reducing agent Hydrazine hydrate (N2H4·H2O) 70 [ml/L]

Complexing agent Tri-sodium citrate (C6H5Na3O7·2H2O) 50 [g/L]

Buffering agent Boric acid (H3BO3) 30 [g/L]

pH adjuster Sodium hydroxide (NaOH) 80 [g/L]

pH of solution 11-13

Plating temperature 70-85 [°C]

Loading ratio 5 [g/L]

Material Characterization. Surface morphologies of the Ni-coated h-BN powders were observed

by scanning electron microscopy (SEM). Phase composition of the coated powders was analyzed by

X-ray diffraction (XRD). After hot-pressed sintering, the Al2O3/(W,Ti)C/h-BN@Ni and

Al2O3/(W,Ti)C/h-BN ceramic disks were cut into bars with an inner-circular cutter. The bars were

ground using a diamond grinding wheel and then polished with diamond abrasive paste to obtain test

specimens with 3 mm×4 mm×36 mm in dimension.

The flexural strength was measured using three-point-bending technique with a span of 20 mm at a

crosshead speed of 0.5 mm/min. The testing forces were parallel to the hot pressing direction. The

Vickers hardness was measured on the polished surface using a Vickers hardness tester with a load of

196 N and loading time of 15 s. The fracture toughness was evaluated by indentation method using the

cracks generated by the Vickers indentations. Five specimens for each ceramic disk were tested and

the average values of flexural strength, Vickers hardness and fracture toughness were taken. SEM was

applied to observe the microstructures of the specimens. The nickel distribution around the h-BN

grains was observed by line scan with energy dispersive spectrometer (EDS).

Dry cutting tests were carried out on a CDE6140A lathe. Both the Al2O3/(W,Ti)C/h-BN@Ni and

Al2O3/(W,Ti)C/h-BN cutting tool materials were made into tool inserts with the type of ISO

SNGN120604 and a chamfered cutting edge (-20°×0.2 mm). The geometries of the tool holder used

were rake angle γ0=-8°, clearance angle α0=8°, inclination angle λs=0° and side cutting edge angle

Kr=45°. The workpiece material was 45# hardened carbon steel with hardness of HRC 30-35. The

machining process was performed at cutting speeds of 90 m/min and 150 m/min, a constant feed rate

of 0.102 mm/rev and a constant depth of cut of 0.2 mm. The average flank wear VB of the ceramic

cutting tools was measured at intervals by an optical microscope until it reached the failure criterion

of 0.3 mm. The wear morphologies were observed by means of SEM.

Results and Discussion

Morphological Analysis of Ni-Coated h-BN Powders. Fig.1 presents SEM micrographs of

Ni-coated h-BN powder and uncoated ones. The uncoated raw h-BN powders have flaky and

polygonal shape with clean and smooth surfaces, as shown in Fig.1(a). By contrast, the Ni-coated

h-BN powder exhibits coarse surface consisting of nanometric particles. The nickel coating is

uniform, compact and complete, as shown in Fig.1(b). Phase composition analysis of Ni-coated h-BN

powders in comparison with raw h-BN powders is shown in Fig.2. It can be seen that only the two

phases of Ni and h-BN are present in the XRD pattern of Ni-coated h-BN powders, indicating that the

h-BN powders were coated by pure nickel which was in crystalline state.

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(a) (b)

Fig.1. SEM micrographs of (a) raw h-BN powders and (b) Ni-coated h-BN powder

Fig.2. XRD patterns of (a) Ni-coated h-BN powders; (b) h-BN powders

Microstructural Characterization of Cutting Tool Materials. Fig.3 shows SEM micrographs

of the polished surface of the two types of self-lubricating ceramic cutting tool materials. The light

gray phases are identified by EDS to be (W,Ti)C, and the deep gray phases are Al2O3 and h-BN. It can

be seen that the microstructure of Al2O3/(W,Ti)C/h-BN@Ni material is more uniform than that of

Al2O3/(W,Ti)C/h-BN material.

(a) (b)

Fig. 3 SEM micrographs of polished surface of self-lubricating ceramic cutting tool materials: (a)

Al2O3/(W,Ti)C/h-BN@Ni; (b) Al2O3/(W,Ti)C/h-BN

As shown in Fig.4(a) and (b), the h-BN grains distribute in the Al2O3/(W,Ti)C/h-BN@Ni material

with orientation and bond closely with the ceramic matrix. By comparison, the h-BN grains distribute

randomly in the Al2O3/(W,Ti)C/h-BN material and some h-BN agglomerates can be found, as shown

in Fig.4(c). The bonding of h-BN grains and the matrix is not so close that some pores may exist in

their interfaces, as can be seen in Fig.4(d). The improvement in microstructure can be explained by

the addition of Ni-coated h-BN powders. On the one hand, Ni-coated h-BN powders can be easier to

disperse than uncoated h-BN powders, resulting in better distribution in the matrix. On the other hand,

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the nickel was in fluid phase at the sintering temperature and played a positive role in promoting

densification. It should be noted that the laminated stack structure of h-BN makes it difficult to

densify the ceramics [13], so there are tiny gaps among some h-BN laminated flakes in

microstructures of the both self-lubricating ceramic cutting tool materials. In addition, fracture modes

of the both cutting tool materials are mixed transgranular fracture and intergranular fracture.

(a) (b)

(c) (d)

Fig.4. SEM micrographs of fracture surfaces of self-lubricating ceramic cutting tool materials: (a)

Al2O3/(W,Ti)C/h-BN@Ni; (b) enlargement of (a); (c)Al2O3/(W,Ti)C/h-BN; (d) enlargement of (c)

In order to observe the distribution of the nickel coating in Al2O3/(W,Ti)C/h-BN@Ni material,

EDS line scan was carried out perpendicular to the h-BN grains in fracture surfaces of the material.

Fig.5 presents distributions of elements boron and nickel across the transverse section of a h-BN grain.

It can be seen that the concentration of element nickel reaches peaks near the both surfaces of the

h-BN grain and declines rapidly to either side. It is indicated that the nickel coating kept in good

condition during preparation processes of Al2O3/(W,Ti)C/h-BN@Ni material and diffused to the

adjacent matrix in the hot pressing.

(a) (b)

Fig.5.EDS line scan patterns perpendicular to a h-BN grain of Al2O3/(W,Ti)C/h-BN@Ni material:

(a) element boron; (b) element nickel

Mechanical Properties of Cutting Tool Materials. As shown in Table 2, the flexural strength,

hardness, and fracture toughness of the Al2O3/(W,Ti)C/h-BN@Ni material were respectively

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increased by about 11%, 5% and 28%, as compared with the corresponding Al2O3/(W,Ti)C/h-BN

material. It is commonly believed that an inhomogeneous distribution of a second phase is harmful to

mechanical properties [14]. These areas where h-BN agglomerates distribute can become the fracture

sources, thus the enhanced flexural strength and hardness of the Al2O3/(W,Ti)C/h-BN@Ni material

may be caused by its more homogeneous microstructure and closely bonded interfaces in virtue of

adding Ni-coated h-BN powders. In addition, ceramic materials can be toughened by incorporating

ductile metal particles in terms of sintering metal coated ceramic powders [15-17]. The incorporated

metal particles can toughen the ceramic materials by the bridging effect and the deflection of cracks.

Therefore, the addition of h-BN@Ni powders should be the reason for the notable fracture toughness

improvement of the Al2O3/(W,Ti)C/h-BN@Ni material.

Table 2. Mechanical properties of self-lubricating ceramic cutting tool materials

Specimen Flexural strength

[MPa]

Vickers hardness

[GPa]

Fracture toughness

[MPa·m1/2

]

Al2O3/(W,Ti)C/h-BN@Ni 568 15.9 5.5

Al2O3/(W,Ti)C/h-BN 510 15.1 4.3

Cutting performances of cutting tools. The average flank wear VB of the two kinds of

self-lubricating ceramic cutting tools in regard to cutting length is illustrated in Fig. 6. It can be seen

that after the same cutting length, the flank wear at the cutting speed of 150 m/min is bigger than that

at the cutting speed of 90 m/min. The flank wear of Al2O3/(W,Ti)C/h-BN@Ni cutting tool is smaller

than that of Al2O3/(W,Ti)C/h-BN cutting tool at cutting speed of 90 m/min and 150 m/min,

respectively. It suggests that wear resistance of the ceramic cutting tool containing Ni-coated h-BN is

higher than that of the ceramic cutting tool containing uncoated h-BN. It was reported that wear rate

of ceramic material when abrasion is a primary wear mechanism is inversely proportional to the

product of its fracture toughness and hardness, i.e., 3/4 1/2

ICK H [8]. Therefore, the better flank wear

resistance of Al2O3/(W,Ti)C/h-BN @Ni cutting tool can be attributed to the higher mechanical

properties especially the hardness and fracture toughness.

Fig. 6. Figure 6 Average flank wear versus cutting length of Al2O3/(W,Ti)C/h-BN@Ni cutting

tool(A) and Al2O3/(W,Ti)C/h-BN cutting tool (B) at different cutting speeds

Fig.7 shows SEM micrographs of the worn flank faces of the two cutting tools with the same

cutting length at cutting speed of 150 m/min. More severe abrasive wear and notch wear can be seen

in the flank wear area of Al2O3/(W,Ti)C/h-BN cutting tool than Al2O3/(W,Ti)C/h-BN@Ni cutting tool.

The cutting edge of Al2O3/(W,Ti)C/h-BN@Ni is in good condition, whereas microchippings are

found near the cutting edge of Al2O3/(W,Ti)C/h-BN. It proves that the cutting tool containing

Ni-coated CaF2 exhibited better abrasive wear resistance and cutting edge chipping resistance

compared with the cutting tool containing uncoated h-BN. In addition, the difference in the amount of

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adhesive substance on the flank wear areas indicates that the self-lubricating ceramic cutting tool with

the addition of h-BN@Ni powders has better anti-adhesive property than the cutting tool with the

addition of uncoated h-BN powders.

(a) (b)

(c) (d)

Fig.7. SEM micrographs of the flank wear area: (a) Al2O3/(W,Ti)C/h-BN@Ni; (b) enlargement of (a);

(c)Al2O3/(W,Ti)C/h-BN; (d) enlargement of (c).

Conclusions

A new type of self-lubricating ceramic cutting tool material was fabricated by adding pure

metalcoated solid lubricant powders. The following are main conclusions of this study:

(1) Ni-coated h-BN powders were prepared by electroless plating process with N2H4·H2O as the

reducing agent. The nickel coating is uniform, compact and complete.

(2) Al2O3/(W,Ti)C/h-BN@Ni self-lubricating ceramic cutting tool material was produced by hot

pressing. The nickel coating kept in good condition and diffused to the adjacent matrix.

(3) By adding Ni-coated h-BN powders, the microstructure of the self-lubricating ceramic cutting

tool material is more homogeneous and the h-BN grains bond more closely with the ceramic matrix as

compared with the material containing uncoated h-BN powders.

(4) The mechanical properties and wear resistance of Al2O3/(W,Ti)C/h-BN@Ni cutting tool

material are evidently better than those of the corresponding Al2O3/(W,Ti)C/h-BN material.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant No. 51575285)

and Science and Technology Development Plan of Shandong Province (Grant No.

2014GGX103001).

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Development of Self-lubricating Ceramic Cutting Tool