Enhanced Strength and Hardness of Graphene Oxide‑Reinforced Titanium Composites via Hot‑Press Sintering

Abstract

Ti matrix composites containing 1–5 wt % graphene oxide (GO) were fabricated by hot‑press sintering under argon. The reaction between Ti and GO produced TiC nanoparticles in situ, while a lamellar GO network was partially retained. Increasing GO loading and sintering temperature raised the TiC content, which in turn improved hardness and strength. The 5 wt % GO composite sintered at 1473 K reached 457 HV (48 % higher than pure Ti) and the 2.5 wt % GO sample at the same temperature exhibited a peak yield stress of 1294 MPa, 63 % above pure Ti. Fractography revealed a quasi‑cleavage mode in the reinforced composites versus ductile fracture in unreinforced Ti. The strengthening arose from grain refinement, solid‑solution effects, and dispersion of TiC and GO.

Introduction

The aerospace sector demands materials that are lightweight yet capable of withstanding extreme loads and temperatures. Titanium matrix composites (TMCs) are attractive candidates due to their high specific strength, excellent wear resistance, and stable high‑temperature performance. Conventional ceramic reinforcements (TiC, SiC, TiB) and SiC fibres enhance stiffness but can embrittle the matrix or introduce anisotropy. Carbon‑based nanomaterials, particularly graphene and its oxidized form, offer a low‑density route to boost strength while preserving toughness. Graphene’s single‑atom thickness and sp² bonding give it a theoretical surface area of 2630 m² g⁻¹ and outstanding electrical, thermal, and mechanical properties. Prior work has shown that adding only 0.5–0.8 wt % graphene derivatives to Al or Cu matrices can increase tensile strength by 20–40 %. However, the benefit plateaus at higher loadings because of nanoparticle agglomeration. Graphene oxide (GO) incorporates oxygen functional groups that enhance dispersion in solvents and may react with the metal matrix to form reinforcing phases. While GO has improved Al and Fe composites, its application to Ti matrices remains underexplored. This study investigates Ti/GO composites fabricated by hot‑press sintering, examining how GO content and sintering temperature influence microstructure, phase formation, and mechanical performance.

Methods and Experimental

Synthesis of Graphene Oxide

GO was produced by a modified Hummers’ route. Graphite (99.95 % purity, <325 mesh) was first intercalated with CrO₃ in HCl, then expanded by H₂O₂ treatment to yield chemically expanded graphite (CEG). Subsequent oxidation with a 9:1 mixture of H₂SO₄/H₃PO₄ and KMnO₄ at 323 K, followed by neutralization with H₂O₂ and HCl, produced GO flakes. Raman, XPS, FT‑IR, and TGA confirmed the presence of oxygenated functional groups and a single‑layer thickness (~1 nm) (Fig. 2).

Composite Powder Preparation

Commercial Ti powder (≥99.9 % purity) was mixed with a GO suspension diluted in 95 % ethanol. The slurry was ultrasonically dispersed for 10 min, dried in a vacuum oven at 333 K, and milled for 10 min to yield a uniform Ti/GO blend (Fig. 4).

Hot‑Press Consolidation

Mixtures were loaded into a 15 mm graphite die and sintered in an argon‑flow furnace at 1073 or 1473 K, with a heating rate of 15 K min⁻¹, 30 min dwell, and 50 MPa pressure. Samples were cooled at <20 K min⁻¹. The resulting compacts were machined into test coupons and polished to 1 µm.

Characterization

Phase analysis was performed by XRD; microstructure by SEM, TEM, and EDS; GO thickness by AFM. Hardness (Vickers, 250 g, 10 s), compressive strength (MTS 858), and thermal conductivity (Netzsch LFA 457) were measured. TGA quantified GO decomposition temperatures.

Results and Discussion

Graphene Oxide Structure

Raman spectra displayed D (1347 cm⁻¹) and G (1582 cm⁻¹) bands with I_D/I_G = 1.46, indicating significant oxidation. XPS C 1s peaks corresponded to sp² C, C–O, C=O, and O–C=O groups. FT‑IR confirmed O–H, C–O–C, and C=O vibrations. TGA showed a major weight loss between 433–493 K, attributable to loss of labile oxygen groups (Fig. 2).

Microstructure and Phase Evolution

SEM images of the Ti/GO powders (Fig. 4) revealed uniform dispersion of GO flakes up to 2.5 wt %; 5 wt % samples exhibited mild agglomeration. After sintering at 1073 K, Ti particles bonded with a strip‑like GO network, but gaps appeared due to limited diffusion of oxygen groups (Fig. 5). EDS mapped TiC nanoparticles at the GO–Ti interfaces, confirming in‑situ reaction. At 1473 K, the matrix became more compact, gaps diminished, and TiC density increased (Fig. 7). TEM of the 2.5 wt % sample showed GO flakes retained as lamellae, TiC particles 20–200 nm, and a high dislocation density at grain boundaries, consistent with Orowan‑type strengthening (Fig. 8).

XRD Analysis

All samples exhibited dominant α‑Ti peaks (100, 002, 101). With GO addition, weak TiO₂ and TiC reflections appeared, growing in intensity at higher GO loadings and temperatures, indicating progressive TiC formation (ΔG ≈ −178 kJ mol⁻¹ at 1073 K). Peak broadening and shift to higher 2θ implied grain refinement (Fig. 9).

Mechanical Properties

Hardness increased from 250 HV (pure Ti) to 457 HV (5 wt % GO at 1473 K), a 48 % improvement. Compressive yield stress rose from 702 MPa (pure Ti) to 1294 MPa (2.5 wt % GO at 1473 K), a 63 % gain. The 5 wt % sample showed a slight decline due to GO clustering (Fig. 10, 11). Elevated sintering temperature enhanced density, TiC formation, and dislocation activity, further boosting strength.

Fractography

Pure Ti displayed dimples indicative of ductile failure. GO‑reinforced composites exhibited quasi‑cleavage planes and microcracks, becoming more pronounced at 1473 K where gaps were reduced (Fig. 12).

Thermal Conductivity

Adding GO reduced conductivity from 22 W m⁻¹ K⁻¹ (pure Ti) to 16 W m⁻¹ K⁻¹ (5 wt % GO) at 873 K, due to low GO thermal conductivity and interfacial gaps. However, conductivity increased with sintering temperature as the microstructure became denser and more GO was reduced to graphene (Fig. 13).

Strengthening Mechanisms

Grain refinement (Hall‑Petch), solid‑solution strengthening by C and O atoms, and dispersion strengthening from TiC and GO were identified as the primary contributors. The high dislocation density, combined with Orowan bowing around GO lamellae, further amplified yield strength.

Conclusions

1. GO disperses uniformly in Ti up to 5 wt %; TiC (20–200 nm) forms in situ via Ti–GO reaction, increasing with GO content and temperature.
2. Hardness and yield strength rise sharply: 457 HV (48 %) and 1294 MPa (63 %) relative to pure Ti at 1473 K.
3. Fracture mode shifts from ductile to quasi‑cleavage; thermal conductivity drops with GO but improves at higher sintering temperatures.
4. Strength gains stem from grain refinement, solid‑solution effects, and dispersion of TiC and GO.

Abbreviations

AFM

Atomic force microscopy

BCC

Body‑centered cubic

CEG

Chemically expanded graphite

CTE

Coefficient of thermal expansion

EDS

Energy‑dispersive spectrometer

FCC

Face‑centered cubic

FT‑IR

Fourier‑transform infrared spectroscopy

GIC

Graphite intercalation compound

GNFs

Graphene nanoflakes

GO

Graphene oxide

HCP

Hexagonal close packing

MMCs

Metal matrix composites

SEM

Scanning electron microscope

TEM

Transmission electron microscopy

TGA

Thermogravimetric analysis

TMCs

Titanium matrix composites

XPS

X‑ray photoelectron spectroscopy

XRD

X‑ray diffraction