Effect of Sandblasting and Surface Mechanical Attrition Treatment on Surface Roughness, Wettability, and Microhardness Distribution of AISI

316L

B. Arifviantoa, Suyitnob, M. Mahardikac

Center for Innovation of Medical Equipments and Devices (CIMEDs)

Department of Mechanical and Industrial Engineering, Universitas Gadjah Mada

Jl. Grafika 2 Yogyakarta 55281, Indonesia

abudi.arif@ugm.ac.id, bsuyitno@ugm.ac.id, cdr064588@gmail.com

Keywords: Sandblasting, Surface mechanical attrition treatment, Surface quality, AISI 316L

Abstract. Surface roughness and wettability determines the stability of bone-implant integration.

Stable implants can be found in those with a rough and hydrophilic surface. Sandblasting and

surface mechanical attrition treatment (SMAT) are among the current techniques to obtain surface

with such typical properties. In addition, both treatments increase mechanical strength of metal

through surface grains refinement. In this paper, the effect of sandblasting and SMAT on surface

roughness, wettability, and microhardness distribution of AISI 316L is discussed. All treatments

were conducted for 0-20 minutes. The result shows a rougher and a more hydrophilic surface on the

sandblasted samples rather than on those with SMAT. A harder surface is yielded by both

treatments, but the SMAT produces a thicker hardened layer.

Introduction

Osseointegration is defined as a direct structural and functional connection between the living bone

and the surface of load-carrying implant without the formation of fibrous tissue [1-3]. It is marked

by the formation of new bone tissues around the implant surface so as to create a bonding between

the implant and the surrounding host tissue. A successful osseointegration can be seen on a strongly

bonded [4] and a stable [1] implant in bone. Investigation by Elias et al. shows a higher implant

removal torque for detaching a rough- and a hydrophilic-implant from the rabbit’s tibia after 12

weeks of implantation. This implies a higher bonding strength between the typical surface and the

surrounding bone tissue that enhances the implant stability [4].

Numerous methods have been introduced to obtain a rough and a hydrophilic implants, including

machining [4], acid-etching [4-6], anodizing [4], sandblasting [4,7,8] and surface mechanical

attrition treatment (SMAT) [9,10]. The last two treatments are not only producing a rough and

hydrophilic surface but also enhancing the strength of material via surface grains refinement [4,11-

13]. However, the prominence of each treatment as compared one another has not yet been

confirmed.

In this work, the surface roughness, wettability, and microhardness distribution on AISI 316L

after sandblasting and SMAT are discussed. The typical steel is selected due to its wide application

for orthopedic implants, including the hip stems that must integrate with the bone tissue [14]. It has

excellent properties in corrosion and oxidation resistance [12] and biocompatibility [15].

Materials and Methods

Samples were prepared from AISI 316L austenitic stainless steel plate with a dimension of 100 × 50

× 4 mm. The samples’ chemical compositions (%wt) are 0.0316 C, 24.3038 Cr, 10.9653 Ni, 1.7477

Mo, 1.2369 Mn, 0.4360 Si, 0.8637 Cu, and 0.0002 S. All samples were polished prior to treatments

in order to obtain uniform surface roughness (Ra ≈ 0.046).

Sandblasting was carried out using 8 kg/cm2 compressed air flow containing silica sands directed

to the sample’s surface. SMAT was conducted using 250 stainless steel balls with a diameter of

Key Engineering Materials Vols. 462-463 (2011) pp 738-743Online available since 2011/Jan/20 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.462-463.738

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 142.103.160.110, University of British Columbia, Kelowna, Canada-18/07/13,09:14:29)

3/16 in (4.7625 mm). The chamber’s vibration in SMAT was generated by a constant shaft rotation

of 1400 rpm. The duration of both treatments was 5, 10, 15, and 20 minutes. The differences on

technical specifications of sandblasting and SMAT are illustrated and presented in Table 1.

Table 1 Illustration and technical specifications of sandblasting and SMAT

Treatment Sandblasting (SB) Surface Mechanical Attrition

Treatment (SMAT)

Illustration

Material Silica sands Stainless steel

Direction of

impact

Single Multiple

Particle/shot

diameter

0.208 mm (average) 4.7625 mm

Particle density 2.19 grams/cm3 (average) 7.46 grams/cm

3

Impact velocity >100 m/s [16] ±20 m/s [16]

Source of particles

motion

Compressed air flow Mechanical vibration

The samples microstructure observation was conducted across the sample sectional area using

Olympus microscope and OptiLab digital microscope for capturing the images. The sample surface

roughness was quantified using Surfcom 120A (Advanced Metrology System). Measurement was

conducted on 30 different locations to obtain the arithmetic medium value (Ra) of each sample. The

surface wettability was quantified through droplet contact angle measurement [17,18]. A distilled-

water droplet was dropped three times at five different locations in each sample. The static droplet

on sample surface was recorded using Canon SX 20 IS digital camera. The microhardness

distribution was determined using Buehler microhardness tester across the samples’ sectional area.

Measurement was conducted from the surface to the deeper location of samples with an indenting

load of 0.5 kgf. The microhardness is represented by the Vickers hardness number.

Results

The microstructures of AISI 316L after 20 minutes of sandblasting and SMAT are presented in

Fig. 1(a) and 1(c)., respectively, while that of the original sample is depicted in Fig. 1(a). All figures

show the typical pattern of austenitic steel microstructures. Reduction of grains size occurs at the

surface layer of samples with sandblasting and SMAT, however, it is not shown obviously in this

study.

Fig. 2 shows the sample roughness after sandblasting and SMAT. Both treatments increase Ra

values; showing their capability of producing a rough surface. Sandblasting produces rougher

surfaces by approximately three folds than SMAT. However, the longer-treatment, either

sandblasting or SMAT, reduces Ra values.

Fig. 3 shows the comparison of droplet contact angles at the surfaces after sandblasting and

SMAT. Both treatments enhance samples wettability as indicated by reduction of droplet contact

angles on the treated samples. Larger droplet contact angles are seen at the surfaces with SMAT

instead of the sandblasted ones. The characteristics of droplet contact angle are also depending on

the duration of treatment. The droplet contact angle of samples with SMAT decreases at the longer-

treated ones. However, extending duration of sandblasting reduces the samples’ droplet contact

angle.

Key Engineering Materials Vols. 462-463 739

Fig. 1 (a) Original sample, (b) Sample treated for 20 minutes with sandblasting, (c) SMAT

0

1

2

3

4

0 5 10 15 20

Duration (min)

Arithmetic Medium Value, Ra (m) Sandblasting SMAT

0

20

40

60

80

100

120

0 5 10 15 20

Duration (min)

Droplet Contact Angle (deg)

Sandblasting SMAT

Fig. 2 Sample surface roughness after

sandblasting and SMAT

Fig. 3 Sample droplet contact angles after

sandblasting and SMAT

1

1.5

2

2.5

3

3.5

0 5 10 15 20

Duration (min)

Microhardness (GPa)

Sandblasting

SMAT

1

1.5

2

2.5

3

3.5

0 0.25 0.5 0.75 1 1.25 1.5

Depth (mm)

Microhardness (GPa)

Sandblasting

SMAT

Control

Fig. 4 Sample microhardness at a distance

of 0.1 mm from the treated surface

Fig. 5 Microhardness distribution across

the samples’ sectional area

Fig. 4 describes the microhardness at a distance of 0.1 mm from the treated surface. Surface

hardening is observed at the samples after sandblasting and SMAT, but early hardening occurs

resulting from the former treatment. After 5 minutes of treatment, a harder surface is shown on the

samples with sandblasting rather than on those with SMAT. However, further increase of hardness

is not observed after 10 minutes treatments.

The microhardness distribution across the samples’ sectional area after 10 minutes of treatments

is described in Fig. 5. The samples’ surface hardness due to both sandblasting and SMAT are equal

at a distance of 0.1 mm, but a thicker hardened layer is observed after the later treatment.

Microhardness decreases by the distance from the treated surface and approaches values of the non-

treated samples.

Discussions

Surface Roughness and Wettability. Surface nanocrystallites can be obtained using several

methods, including by (1) consolidation of nanocrystalline powders, (2) physical, chemical, and

electrochemical deposition, (3) crystallization of amorphous materials, and (4) severe plastic

deformation (SPD) [19]. Unlike to the others, SPD produces refined grains without introducing

740 Fracture and Strength of Solids VII

certain substances into the material [13,20]. Sandblasting and SMAT are among the methods based

on SPD to produce surface nanocrystallites.

The current experiment shows that the particles or shots impact creates defects at the sample

surface and results in the formation of a rough surface. Sandblasting yields a rougher surface than

SMAT which may be due to the particle properties used in treatment. The rough and irregular

shaped particles, even those with sharp-edges, in sandblasting may cause severe pitting on surface.

In addition, the high velocity of particles bombardment may also contribute on the formation of

deeper pits at the surface. Meanwhile, SMAT utilizes milling balls with smooth surface and

relatively low impact velocity that yields less rough surfaces than the sandblasted ones.

The sample roughness is also influenced by the duration of treatments. The less rough surfaces

are found at the longer-treated samples, either after sandblasting and SMAT. A longer treatment

means to a more impacts and thus yields more nanocrystallites at the surface. The presence of more

nanocrystallites therefore reduces surface roughness.

In this work, sandblasting and SMAT yield different level of wettability. Lower droplet contact

angles are observed on the sandblasted surfaces, instead of those with SMAT; indicating that

sandblasting produces a more hydrophilic surface than its counterpart. The surface hydrophilicity

may be attributed by its roughness, in which the sandblasted surface is rougher than that with

SMAT. This is in good agreement with the previous studies that reveal an inverse relationship

between surface roughness and droplet contact angle [4,9,10]. The underlying mechanism of such

relationship has been well explained by Uelzen and Muller, in which the droplet contact angle on a

rough surface is given by considering the ratio between the apparent surface and its projection on a

flat surface [21].

Extending SMAT duration increases droplet contact angle following the reduction of Ra value,

but a similar pattern is not shown by that in the sandblasted surface. The droplet contact angle on

the sandblasted surface decreases by the reduction of Ra value. Difference on the pattern or texture

of surface after sandblasting and SMAT may contribute to this result [22], which is related to the

use of different shots or particles geometry and properties. Further investigation on this case needs

to be performed.

Protein adsorption is the most immediate event that occurs after implantation of foreign materials

[23]. The presence of adsorbed proteins such as vitronectin and fibronectin are crucial for initiating

osseointegration [19,23,24]. The role of surface roughness and wettability on protein adsorption

onto the implant surface has been discussed in many publications. The hydrophilic surface enhances

vitronectin performance in the competition with other proteins to adsorb the implant surface. In

addition, the functionality of fibronectin in cell adhesion is improved on such typical surface [17].

Other publication reveals that a hydrophobic surface tends to be more likely adsorbed by albumin

and become harder to be replaced by the extracellular matrix (ECM) than in a hydrophilic one. The

presence of ECM determines cell adhesion, spreading, geometry and apoptosis [25] whereas

albumin blocks the cell adhesion on implant surface [24]. A higher concentration of albumin than

any other useful proteins for osseointegration makes this substance adhere early to the implant

surface. However, a rough surface enhances the affinity of fibronectin so that it is capable of

replacing albumin from the surface [24,26].

The current study shows that sandblasting and SMAT produce surface roughness of Ra = 2.4-

2.6 µm and Ra = 0.7-0.9 µm, respectively. The droplet contact angle measurement reveals θ = 60-

76° and θ = 78-84° at the surfaces with sandblasting and SMAT, respectively. Uelzen and Muller

classify the hydrophilic surfaces as those with θ < 90° [21], but Vogler establishes θ < 65° [27].

According to this classification, it seems that reducing droplet contact angle using sandblasting may

preferable for producing hydrophilic surface instead of SMAT.

Microhardness Distribution. Both sandblasting and SMAT transforms the original coarse

surface grains into the finer ones [12,29] by impact of shots or particles onto the surface of

materials. The grain refining mechanisms in stainless steel via SMAT are in the following route: (1)

formation of mechanical twinning, (2) grain subdivision by twin-twin intersection and (3) formation

of randomly oriented nanocrystallites [11,29,30]. Surface grains with nanometer scale are formed in

Key Engineering Materials Vols. 462-463 741

the last stage of the route. Investigation by Roland et al. show unidirectional parallel twins and

twin-twin intersection at a depth of 200 and 50 µm, respectively, from the AISI 316L surface after

SMAT. In addition, a decreasing hardness at the sample’s subsurface by the distance from the

surface to the deep matrix of material is indicated. This implies that hardening mechanism occurs

following the formation of nanocrystallites during the SMAT. The microhardness distribution of

samples with SMAT in the current study shows a similar pattern with the result obtained by Roland

et al. [11]. Moreover, the samples’ microhardness of 3 GPa is observed at a distance of 0.1 mm

from the treated surface in both studies. Both are consistent with Hall-Petch theory that reveals an

inverse relationship between the hardness and the grain size. Moreover, the hardening via SMAT is

also attributed by the presence of residual stress [11].

Surface hardening on the sandblasted samples may be resulted from grains reduction as well as

in SMAT. However, other factors such as the formation of α’-martensite and the presence of

residual stress at the surface should be also taken into consideration. The α’-martensite is harder

than the austenite and the presence of this substance is detected at the subsurface of sandblasted

AISI 316LVM [28]. The residual stress exists resulting from the indentations or dimples of shot

particles that are imparted to the sample’s surface. The shot impact deform the surface plastically,

but the surrounding elastic material attempts to return the yield stress to its initial value.

Consequently, a residual compressive stress takes place and increases the surface hardness by cold-

work process [31]. The surface microhardness of the sandblasted AISI 316L in this study is slightly

lower than that found by Multigner et al. [28], which is probably due to the differences of technical

parameters used in treatment, including the particle size and the air pressure for blasting the

particles.

The surface hardness and thickness of hardened layer at the subsurface are determined by the

magnitude of impact load and duration of peening [32]. In this experiment, a harder surface is seen

on the longer-processed samples, either by sandblasting or SMAT as the formation of more

nanocrystallites. However, a constant hardness is observed after 10 minutes of treatment, which is

probably related to the grains size saturation. SMAT may deliver a slightly greater impact load than

sandblasting as indicated by a thicker hardened layer after such treatment. Such enhancement after

both treatments may indicate that SMAT and sandblasting are useful for increasing the strength and

fatigue life of implants [11,33].

Conclusions

Sandblasting and SMAT utilize a similar method to produce a rough and a hydrophilic surface, in

addition to enhance the strength of metallic implants. In this study, however, sandblasting and

SMAT yields different results. A rougher and a more hydrophilic surface are found in the

sandblasted samples instead of the SMAT ones. Early surface hardening is also yielded by

sandblasting. But, SMAT is more capable of enhancing the thickness of hardened layer at the

surface, which is crucial for the improvement of strength and service lifetime of metallic implants.

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Key Engineering Materials Vols. 462-463 743

Fracture and Strength of Solids VII 10.4028/www.scientific.net/KEM.462-463 Effect of Sandblasting and Surface Mechanical Attrition Treatment on Surface Roughness,

Wettability, and Microhardness Distribution of AISI 316L 10.4028/www.scientific.net/KEM.462-463.738